feat: add send-message step plugin base and Go implementation

extended/docs/proposals/0004-send-message-step-plugin/implementation_checklist.md

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+# 0004 Implementation Checklist
+
+Update this as implementation teaches us things.
+
+## Baseline
+
+- [x] Plugin work lives under `extended/plugins/send-message`.
+- [x] The plugin subtree does not rely on repo resources outside itself.
+- [x] Slack credentials stay local-only and out of git.
+- [x] The host does not own channel lookup, Secret reads, or Slack API calls.
+
+## Phase 0: read and pin the seams
+
+- [x] Read proposal 0004 again before starting code.
+- [x] Re-read the `send-message` section in proposal 0002 spec.
+- [x] Read the current StepPlugin host runtime and e2e harness seams.
+- [x] Record any host-side gap found during that read.
+
+## Phase 1: create the standalone subtree
+
+- [x] Create `extended/plugins/send-message/`.
+- [x] Add plugin-owned runtime source there.
+- [x] Add plugin-owned image build there.
+- [x] Add plugin-owned tests there.
+- [x] Add plugin-owned CRD manifests there.
+- [x] Add plugin-owned RBAC manifests there.
+- [x] Add plugin-owned smoke assets or smoke helper there.
+
+## Phase 2: implement the runtime
+
+- [x] Implement `POST /api/v1/step.execute`.
+- [x] Enforce bearer-token auth from `/var/run/kargo/token`.
+- [x] Return `403` on bad auth.
+- [x] Reject unsupported methods cleanly.
+- [x] Parse the v1 step-config subset.
+- [x] Resolve `MessageChannel` from the Project namespace.
+- [x] Resolve `ClusterMessageChannel` from the system-resources namespace.
+- [x] Resolve referenced Secrets from the correct namespace.
+- [x] Read Slack token from Secret key `apiKey`.
+- [x] Send the Slack message from the plugin.
+- [x] Return `slack.threadTS` output.
+
+## Phase 3: own the OSS CRDs and RBAC
+
+- [x] Add Slack-only `MessageChannel` CRD manifest.
+- [x] Add Slack-only `ClusterMessageChannel` CRD manifest.
+- [x] Keep the API group `ee.kargo.akuity.io/v1alpha1`.
+- [x] Add plugin-owned RBAC manifests for channel and Secret reads.
+- [x] Keep RBAC setup out of host code unless a real host gap is proven.
+
+## Phase 4: tests inside the subtree
+
+- [x] Add runtime tests for auth.
+- [x] Add runtime tests for namespaced channel lookup.
+- [x] Add runtime tests for cluster-scoped channel lookup.
+- [x] Add runtime tests for referenced Secret lookup.
+- [x] Add runtime tests for Slack request shaping.
+- [x] Add runtime tests for `slack.channelID` override.
+- [x] Add runtime tests for `slack.threadTS` override and output.
+- [x] Add runtime tests for missing channel or Secret failures.
+- [x] Add runtime tests for Slack API failure handling.
+
+## Phase 5: local smoke path
+
+- [x] Build the plugin image from `extended/plugins/send-message`.
+- [x] Load the image into the local kind cluster.
+- [x] Keep kube access on an isolated `KUBECONFIG`.
+- [x] Keep the user's existing kube context untouched.
+- [x] Install plugin CRDs and RBAC.
+- [x] Generate and install the StepPlugin `ConfigMap`.
+- [x] Inject a local-only Slack token into a cluster `Secret`.
+- [x] Create a test `MessageChannel` or `ClusterMessageChannel`.
+- [x] Run a `Stage` with `uses: send-message`.
+- [x] Prove promotion success in-cluster.
+- [x] Prove `slack.threadTS` output is populated.
+- [x] Verify the message appeared in the target Slack channel.
+
+## Phase 6: repo harness integration
+
+- [x] Extend `extended/tests/e2e_stepplugins.sh` to support the real
+      `send-message` smoke path.
+- [x] Keep the committed harness credential-free.
+- [x] Gate Slack smoke on a local env var for the token.
+- [x] Keep any non-`extended/` edit to a tiny hook only, if one is needed.
+
+## Phase Post-Green: Minimize Diff Of Files Outside ./extended Against Kargo Upstream
+
+- [x] Fetch `upstream`.
+- [x] Review every edited file outside `extended/`, if any, against
+      `upstream/main`.
+- [x] Move more logic behind `extended/` helpers if that shrinks the outside
+      diff safely.
+- [x] Re-run matching tests after each cleanup pass.
+- [x] Stop only when no obvious outside-`extended/` shrink remains.

extended/docs/proposals/0004-send-message-step-plugin/implementation_notes.md

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+# 0004 Implementation Notes
+
+- Plugin subtree:
+  - `extended/plugins/send-message/`
+  - standalone `go.mod`
+  - standalone `Dockerfile`
+  - standalone `plugin.yaml`
+  - standalone CRDs and RBAC under `manifests/`
+  - standalone smoke helper under `smoke/`
+
+- Runtime contract implemented in the plugin:
+  - `POST /api/v1/step.execute`
+  - bearer auth from `/var/run/kargo/token`
+  - direct Kubernetes API reads from projected service-account credentials
+  - direct Slack API call from the plugin
+  - encoded `json`, `yaml`, and `xml` payload support
+
+- V1 scope shipped:
+  - `send-message`
+  - `MessageChannel`
+  - `ClusterMessageChannel`
+  - Slack only
+  - no SMTP
+
+- Secret and channel rules implemented:
+  - `secretRef.name` resolves in the Project namespace for `MessageChannel`
+  - `secretRef.name` resolves in the system-resources namespace for
+    `ClusterMessageChannel`
+  - Slack token key is `apiKey`
+  - `spec.slack.channelID` is required in CRD schema
+
+- Smoke harness knobs:
+  - `STEPPLUGIN_SEND_MESSAGE_SMOKE=true`
+  - `STEPPLUGIN_SEND_MESSAGE_CHANNEL_ID=<channel-id>`
+  - `STEPPLUGIN_SEND_MESSAGE_SLACK_API_KEY=<token>`
+- Plugin-local smoke entrypoint:
+  - `extended/plugins/send-message/smoke/smoke-test.sh`
+  - repo harness calls that script rather than owning the full orchestration
+
+- Host gap found during smoke:
+  - the StepPlugin auth-token mount was not readable by non-root sidecars
+  - fix was in `extended/pkg/stepplugin/agent/command_bridge.go`
+  - auth directory mode is now `0755`
+  - auth file mode is now `0444`
+
+- Smoke-result note:
+  - plugin response includes `output.slack.threadTS`
+  - Promotion state observed in-cluster stored the value under
+    `status.state.step-1.slack.threadTS`
+  - the smoke harness accepts either that path or
+    `status.stepExecutionMetadata[0].output.slack.threadTS`
+
+- Encoded-message note:
+  - when `encodingType` is set, `config.slack.channelID` and
+    `config.slack.threadTS` are ignored
+  - encoded body uses Slack-native field names such as `channel` and
+    `thread_ts`
+  - if encoded body omits `channel`, plugin fills it from channel resource
+    `spec.slack.channelID`
+
+- XML note:
+  - no public exact upstream XML example was found
+  - this implementation treats XML as a Kargo-owned alternate serialization of
+    the same Slack payload object used for `json` and `yaml`
+  - root name is ignored
+  - repeated sibling names become arrays
+
+- Non-plugin repo fix found during smoke:
+  - `pkg/cli/cmd/promote/promote.go` assumed wrapped REST payloads
+  - local smoke showed direct-object and direct-list payloads
+  - decoder helpers now accept both shapes

extended/docs/proposals/0004-send-message-step-plugin/implementation_plan.md

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+# 0004 Implementation Plan
+
+## Goal
+
+- Deliver a real Slack-only `send-message` StepPlugin under
+  `extended/plugins/send-message`.
+- Keep that subtree self-contained enough to behave like its own Git repo.
+- Do not rely on code, files, build helpers, tests, or other resources outside
+  `extended/plugins/send-message` for the plugin implementation itself.
+- Use the StepPlugin host slice from proposal
+  [0002-kargo-executor-plugins-from-argo-workflows](../0002-kargo-executor-plugins-from-argo-workflows/proposal.md)
+  without widening host responsibilities.
+- Keep channel lookup and referenced Secret reads in the plugin, not the host.
+- Use local-only Slack credentials for smoke testing. Do not commit any real
+  token or local token source.
+- Make the plugin match the documented Slack subset, including encoded-message
+  behavior, except for SMTP which stays out of scope.
+
+## Implementation Shape
+
+- Put all plugin-owned source, manifests, tests, and smoke assets under:
+  - `extended/plugins/send-message/`
+- Treat that subtree as a standalone plugin repo:
+  - own runtime source
+  - own image build
+  - own CRDs
+  - own RBAC manifests
+  - own tests
+  - own smoke-test helpers
+  - own smoke entrypoint script
+- It is acceptable for repo-level smoke harness code outside that subtree to
+  call into the subtree's smoke entrypoint.
+- It is not acceptable for plugin runtime code inside that subtree to import or
+  shell out to helpers elsewhere in this repo.
+
+## Plugin Runtime
+
+- Build a plugin-owned runtime image from `extended/plugins/send-message/`.
+- Implement `POST /api/v1/step.execute`.
+- Enforce the StepPlugin bearer-token contract:
+  - read `/var/run/kargo/token`
+  - require `Authorization: Bearer ...`
+  - return `403` on bad auth
+- Read Kubernetes credentials from the mounted service account projection when
+  present.
+- Use direct Kubernetes API reads from the plugin:
+  - `MessageChannel` in the Project namespace
+  - `ClusterMessageChannel` cluster-scoped
+  - referenced `Secret` in the Project namespace or system-resources namespace
+- Call Slack from the plugin runtime, not through the host.
+
+## V1 Behavior
+
+- Scope:
+  - `send-message`
+  - `MessageChannel`
+  - `ClusterMessageChannel`
+  - Slack only
+- Do not implement SMTP.
+- Support this step-config subset in v1:
+  - `channel.kind`
+  - `channel.name`
+  - `message`
+  - `encodingType`
+  - `slack.channelID`
+  - `slack.threadTS`
+- Support this output subset in v1:
+  - `slack.threadTS`
+- Plaintext behavior:
+  - `message` is sent as Slack `text`
+  - `slack.channelID` overrides channel resource `spec.slack.channelID`
+  - `slack.threadTS` is sent as Slack `thread_ts`
+  - output `slack.threadTS` is `config.slack.threadTS` if set, else Slack
+    response `ts`
+- Encoded-message behavior:
+  - support `json`, `yaml`, and `xml`
+  - when `encodingType` is set, treat the body as the Slack payload object
+  - ignore `config.slack.channelID` and `config.slack.threadTS`
+  - if encoded body omits `channel`, fill it from channel resource
+    `spec.slack.channelID`
+  - if encoded body sets `thread_ts`, return that value in output
+  - otherwise return Slack response `ts` in output
+- XML behavior:
+  - XML is a Kargo-owned alternate serialization of the same Slack payload
+    object used for `json` and `yaml`
+  - root element name is ignored
+  - attributes become keys
+  - repeated sibling element names become arrays
+  - nested elements become nested objects
+- `MessageChannel` lookup rules:
+  - resource kind is `MessageChannel`
+  - resource namespace is the Project namespace from the step context
+  - `secretRef.name` resolves in the same namespace
+  - `spec.slack.channelID` is required
+  - Slack token key is `apiKey`
+- `ClusterMessageChannel` lookup rules:
+  - resource kind is `ClusterMessageChannel`
+  - `secretRef.name` resolves in the system-resources namespace configured for
+    the plugin install
+  - `spec.slack.channelID` is required
+  - Slack token key is `apiKey`
+
+## CRDs And RBAC
+
+- The plugin subtree owns the OSS CRD manifests for:
+  - `MessageChannel`
+  - `ClusterMessageChannel`
+- Use the existing Akuity API group:
+  - `ee.kargo.akuity.io/v1alpha1`
+- Keep schemas minimal but structurally valid for the Slack-only slice.
+- Ship plugin-owned RBAC manifests under the subtree.
+- For the local smoke path:
+  - bind only the test Project default `ServiceAccount`
+  - grant channel reads and referenced Secret reads needed by the plugin
+  - keep that setup in the smoke assets or smoke helper, not in host code
+
+## Tests
+
+- Keep plugin tests under `extended/plugins/send-message/`.
+- Cover at least:
+  - bearer-token auth
+  - `MessageChannel` lookup
+  - `ClusterMessageChannel` lookup
+  - referenced Secret lookup
+  - Slack request shaping for plain text
+  - `slack.channelID` override
+  - `slack.threadTS` override
+  - `slack.threadTS` output
+  - encoded-body behavior ignores `config.slack.*`
+  - `xml` decode path
+  - failure path when channel or Secret is missing
+  - failure path when Slack returns an error
+
+## Smoke Test
+
+- Put the primary smoke script in:
+  - `extended/plugins/send-message/smoke/smoke-test.sh`
+- That script assumes Kargo is already available and takes its inputs from env.
+- Keep the committed smoke path credential-free.
+- Expect a local env var for the Slack API token during local smoke testing.
+- Build the plugin image from `extended/plugins/send-message/`.
+- Load that image into the local kind cluster without touching the user's
+  global kube context.
+- Install plugin CRDs, RBAC, and generated StepPlugin `ConfigMap`.
+- Create either a `MessageChannel` or `ClusterMessageChannel` test resource and
+  the referenced Secret in-cluster from the local-only token source.
+- Run a `Stage` with `uses: send-message`.
+- Prove in-cluster success from `Promotion` status, including non-empty
+  `slack.threadTS`.
+- For interactive local verification, also confirm the message appeared in the
+  target Slack channel.
+- The repo harness may call this script at the right point in e2e flow, but the
+  smoke orchestration lives in the plugin subtree.
+
+## Host Changes
+
+- Prefer zero host changes.
+- If the plugin proves a host gap, keep any host fix thin and behind
+  `extended/`.
+- Do not move channel lookup, Secret reads, or Slack API calls into the host.
+- Host gap actually found:
+  - non-root sidecars could not read `/var/run/kargo/token`
+  - keep the fix thin in `extended/pkg/stepplugin/agent/command_bridge.go`
+  - auth directory mode must permit traversal by non-root sidecars
+  - auth file mode must permit read by non-root sidecars
+
+## Phase Post-Green: Minimize Diff Of Files Outside ./extended Against Kargo Upstream
+
+1. Get the feature green first.
+2. Fetch `upstream/main`.
+3. Review every edited file outside `extended/`, if any.
+4. Move logic behind `extended/` helpers where that shrinks the conflict
+   surface.
+5. Re-run the matching tests after each cleanup pass.
+6. Stop when no obvious helper extraction or edit-block reduction remains.

extended/docs/proposals/0004-send-message-step-plugin/proposal.md

@@ -20,8 +20,6 @@ Date: 2026-03-24
   `extended/plugins/send-message`.
 - Use that subtree to test whether a third party can implement the plugin
   without merge permissions in the main repo.
-- Match the public Kargo `send-message` step shape for the Slack subset as
-  closely as public docs allow.
 - Keep channel lookup and referenced Secret reads in the plugin, not the host.
 - Do not implement SMTP in this slice.