Add AGENTS.md bridge to the private planning steward

AGENTS.md

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+# ACP Java SDK Agent Instructions
+
+This public repository owns code, tests, Maven builds, releases, shipped contracts, and public
+documentation. Private planning and control state are authoritative in
+`/home/mark/projects/acp-java-steward`; read its `BINDING.md` before planning or executing work.
+
+This repository belongs to the `agentclientprotocol` organization and is maintained, not owned,
+from that steward. Repository settings, secrets, branch protection and organization membership are
+the organization's; do not plan work that assumes them.
+
+Use `./mvnw`, never `mvn`. The normal gate is `./mvnw clean verify`, which runs the unit tests and
+the `*IT` integration classes through Failsafe. Before 0.15.0 the integration tests never ran, so
+"tests pass" on older history did not include them.
+
+Releases happen only by dispatching the organization's `release.yml` workflow; a local
+`-Prelease deploy` is not a path, because the local publishing token is not entitled to the
+`com.agentclientprotocol` namespace. The workflow's release commit bumps the six `pom.xml` files and
+tags; it does not touch `CHANGELOG.md`. Keep the changelog current by hand, in the same change
+that earns the entry.
+
+The wire format is the contract. Polymorphic types write their discriminator exactly once
+(`Access.WRITE_ONLY` on the record component, `visible = true` on the type info); a duplicate JSON
+key passes Jackson-based peers and breaks strict ones. Unstable protocol methods carry
+`@UnstableAcpApi`; a method without it is a stable promise.
+
+Client-session notifications are delivered in order through a single sink drained by `concatMap`,
+and `closeGracefully()` waits for that drain, bounded by the request timeout, while `close()`
+interrupts. Sync clients always have async handlers, because every sync consumer is wrapped in
+`subscribeOn`. Keep both properties when touching `AcpClientSession` or a transport, and add the
+test that would have caught their loss.
+
+Fixtures and reproductions come from the protocol's own schema or a real peer, not from memory of
+what the schema said. The canonical spec is `agentclientprotocol/agent-client-protocol`; check the
+released schema version before adding or changing a method.
+
+The project is Apache-2.0 with the verbatim license text; `LICENSE` and `NOTICE` ship in every
+artifact. Commit messages contain no AI attribution.
+
+Do not copy private planning, current-action, roadmap, checkpoint, or dirty-tree state into public
+files. The ignored `plans/` tree in a checkout is transition material, not public authority.