feat: add python gr2 platform adapter and sync surfaces

gr2/docs/ASSESS-SYNC-ADVERSARIAL-SPECS.md

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+# Assess Sync Adversarial Specs
+
+Artifact 2 for the Sprint 20 sync lane.
+
+This document lists the failure-first specs the Python `gr2 sync` implementation
+must satisfy before `sync run` is allowed to mutate workspace state.
+
+## 1. Missing Spec
+
+Preconditions:
+- workspace has no `.grip/workspace_spec.toml`
+
+Trigger:
+- `gr2 sync status <workspace>`
+
+Expected:
+- command fails immediately
+- error points to `gr2 workspace init`
+- no cache, repo, lane, or event state is written
+
+## 2. Partial Clone Failure
+
+Preconditions:
+- spec declares 3 repos
+- repo A and B are reachable
+- repo C remote is invalid or unavailable
+
+Trigger:
+- `gr2 sync run`
+
+Expected:
+- planner marks A/B runnable and C failing before execution starts
+- execution stops on C if C is in the same phase batch
+- result reports:
+  - A/B success or skipped state explicitly
+  - C as failure with repo-scoped error payload
+- no successful repo update is silently rolled back
+- event outbox records partial progress and terminal failure
+
+Invariant:
+- sync never reports all-green on partial workspace failure
+
+## 3. Dirty Shared Repo
+
+Preconditions:
+- shared repo checkout exists
+- uncommitted changes in repo root
+
+Trigger:
+- `gr2 sync status`
+
+Expected:
+- issue `dirty_shared_repo`
+- issue blocks sync
+- planner does not schedule branch movement or fetch-dependent mutation through
+  the dirty checkout
+
+Invariant:
+- dirty state wins over convenience
+
+## 4. Dirty Lane Checkout During Sync
+
+Preconditions:
+- lane checkout exists
+- lane repo has uncommitted changes
+
+Trigger:
+- `gr2 sync status`
+
+Expected:
+- issue `dirty_lane_repo`
+- issue blocks sync
+- planner may still inspect other repos, but lane mutation is blocked
+
+Invariant:
+- lane-local work is never overwritten by workspace sync
+
+## 5. Conflicting Branch States Across Repos
+
+Preconditions:
+- lane spans repos `app`, `api`, `premium`
+- expected branch is `feat/auth`
+- `app` is on `feat/auth`
+- `api` is behind remote
+- `premium` is on a different local branch
+
+Trigger:
+- `gr2 sync status`
+
+Expected:
+- planner reports repo-scoped branch inspection operations
+- branch divergence appears as explicit sync issue, not implicit correction
+- no automatic branch checkout/rebase in status mode
+
+Invariant:
+- branch alignment must be explicit before mutation
+
+## 6. Shared Cache Path Conflict
+
+Preconditions:
+- `.grip/cache/repos/<repo>.git` exists
+- path is not a bare git directory
+
+Trigger:
+- `gr2 sync status`
+
+Expected:
+- issue `cache_path_conflict`
+- sync blocks
+- planner does not attempt to reuse or overwrite the invalid cache path
+
+## 7. Invalid Repo Hook Config
+
+Preconditions:
+- shared repo has `.gr2/hooks.toml`
+- file does not parse or violates schema
+
+Trigger:
+- `gr2 sync status`
+
+Expected:
+- spec validation fails before sync planning proceeds
+- sync status returns blocked with the hook validation error included
+
+Invariant:
+- repo hook errors fail fast at plan time
+
+## 8. Sync During Active Edit Lease
+
+Preconditions:
+- lane has an active `edit` lease
+- lane repo is otherwise clean
+
+Trigger:
+- `gr2 sync run --lane <lane>`
+
+Expected:
+- sync refuses lane mutation for the leased lane
+- non-lane workspace inspection may still succeed
+- result clearly distinguishes lease-blocked lanes from unrelated workspace
+  status
+
+Invariant:
+- sync does not tunnel through active edit occupancy
+
+## 9. Concurrent Sync From Two Worktrees
+
+Preconditions:
+- same workspace available from two operator shells
+- both invoke sync against overlapping repos
+
+Trigger:
+- `gr2 sync run` concurrently
+
+Expected:
+- shared mutable resources use explicit lock discipline
+- losing side returns machine-readable contention error
+- no cache corruption, no partially-written apply metadata
+
+Invariant:
+- concurrency failure is reported, not hidden as random repo damage
+
+## 10. Platform Backend Failure
+
+Preconditions:
+- `PlatformAdapter` backend is GitHub via `gh`
+- `gh` auth is invalid or the command times out
+
+Trigger:
+- sync planner tries to refresh PR/check state
+
+Expected:
+- repo/local sync inspection still reports local status
+- platform-dependent operations are marked degraded or failed
+- failure is explicit in the result payload
+
+Invariant:
+- adapter failure must not masquerade as clean workspace state