FN-5889: suppress post-done continuation wedges

Keep completed post-done continuation errors from wedging review tasks in a failed state.

- route step-session non-continuable continuation errors through the executor recovery path before marking tasks failed
- add self-healing recovery and run-audit events for in-review tasks already wedged by post-done non-continuable errors
- extend reliability coverage and architecture/backstop docs for the new post-done wedge handling

Files changed:
 AGENTS.md                                          |   1 +
 docs/architecture.md                               |   3 +-
 packages/engine/src/__tests__/reliability-interactions/post-done-continuation-no-wedge.test.ts        | 131 ++++++++++++++++++++-
 packages/engine/src/executor.ts                    |  11 +-
 packages/engine/src/run-audit.ts                   |   2 +
 packages/engine/src/self-healing.ts                |  97 ++++++++++++++-
 6 files changed, 236 insertions(+), 9 deletions(-)

Fusion-Task-Id: FN-5889

Fusion-Task-Lineage: c22716f2-cddb-4c98-ac87-0282017e7c82

docs/architecture.md

@@ -1737,7 +1737,7 @@ This section preserves the detailed lifecycle/self-healing contracts that were f
 - **Scheduler overlap priority/age guard (FN-5325)**: with `groupOverlappingFiles=true`, scheduler now defers a lower-priority (or younger same-priority) candidate when an overlapping queued todo task exists, preserving priority→age→task-id order for overlap serialization without preempting in-progress work. If the inversion is against an already-running lower-priority blocker, scheduler still defers and emits `scheduler:overlap-priority-inversion` once per (candidate, blocker, pass).
 - **Empty-commit refusal + early empty-own-diff finalize (FN-5345/FN-5377)**: Fusion task worktrees install a `prepare-commit-msg` hook that refuses `git commit --allow-empty` and other zero-staged-diff commits, preventing verification-only tasks from manufacturing empty handoff commits that defeat the merger's no-op classifier. The hook allows legitimate empty-tree paths (amend, merge, squash, cherry-pick, revert, rebase). Amend detection tokenizes the parent process command line (`ps -o args=` with `/proc/$PPID/cmdline` fallback for Alpine/busybox) and stops at the first message-supplying flag (`-m`/`-F`/`--message`/`--file`) so a commit message containing the substring `--amend` cannot bypass the guard. In `aiMergeTask`, an early empty-own-diff fast-path runs BEFORE any reuse-handoff acquisition: when integration mode is `reuse-task-worktree`, the branch exists, `git rev-list --count <mergeTarget>..<branch>` is > 0, and `git diff --quiet <mergeBase>..<branch>` exits 0, the task auto-finalizes as no-op with `mergeDetails.noOpMerge: true` and emits `task:auto-recover-finalize-already-on-main` with `reason: "empty-own-diff-early-fast-path"`. The fast-path best-effort removes the stranded worktree (FN-4811 same-task/foreign-owner guard) and deletes the `fusion/<id>` branch so empty-own-diff residuals do not accumulate. This unsticks tasks where a stale empty handoff commit combined with drifted worktree↔branch mapping would otherwise wedge the handoff gate with `registered-branch-mismatch`. The explicit `cwd-integration-branch` mode is unchanged (`cwd-main` remains a deprecated alias normalized to it). `classifyOwnedLandedEvidence` also detects empty-own-diff (aheadCount > 0, zero net diff) and returns `proven-no-op` so downstream self-healing and post-handoff finalize paths benefit too. Additionally, merger's reuse-fallback path now consults `git worktree list --porcelain` before creating a new worktree: extant usable registrations of `fusion/<id>` are reused directly (rather than blindly `git worktree add -f` producing a duplicate registration), and stale registrations are pruned first. The direct-reuse shortcut is guarded by FN-4811 (refuses paths owned by a different task in `activeSessionRegistry`) and FN-4954 (skipped when `recycleWorktrees=true` with a pool attached, so `WorktreePool.acquire` lease bookkeeping stays consistent). Two audit subtypes — `merge:reuse-fallback-pruned-stale-registration` and `merge:reuse-fallback-reused-existing-registration` — replace the prior overloading of `merge:reuse-fallback-new-worktree` for these cases.
 - **In-review branch-binding self-heal (FN-5083)**: `reconcile-in-review-branch-rebind` runs after `reconcile-task-worktree-metadata` and before `reclaim-stale-active-branches`. It restores `task.branch` (and clears `task.worktree` for fresh acquisition) for `in-review` tasks when exactly one case-insensitive `fusion/<id>` candidate branch has unique commits versus the integration base. Ambiguous candidates emit `task:auto-rebind-skipped` (`reason: "ambiguous-candidates"`) and are never auto-resolved. Branch construction across executor/worktree-pool/worktree-acquisition/merger/self-healing canonicalizes to lowercase via `canonicalFusionBranchName`; `fn_task_done` wrong-branch checks now auto-canonicalize case-only mismatches and emit `branch:auto-canonicalize-case`.
-- **In-review is terminal-until-merged under `autoMerge: false` (FN-5147)**: when a project sets `settings.autoMerge: false`, `in-review` is the intended resting state until a human merges the PR. No lifecycle-mutating self-healing sweep (`reclaimSelfOwnedBranchConflicts`, `recoverGhostReviewTasks`, `recoverStaleIncompleteReviewTasks`, `recoverInterruptedMergingTasks`, `recoverStuckMergeDeadlocks`, `recoverMissingWorktreeReviewFailures`, `recoverPartialProgressNoTaskDoneFailures`, `recoverCompletionHandoffLimbo`, `recoverMergeableReviewTasks`, `recoverMergedReviewTasks`, `recoverAlreadyMergedReviewTasks`, `recoverOrphanOnlyScopeViolations`, `recoverForeignOnlyContaminatedInReviewTasks`, `recoverReviewTasksWithFailedPreMergeSteps`, `finalizeNoOpReviewTasks`, `surfaceInReviewStalls`, `surfaceInReviewStalled`) may move the task out of `in-review`, mark it `paused`/`failed`, or re-enqueue it for execution. Scoped FN-5819 exception: shared-group members (`branchContext.assignmentMode === "shared"`) are still allowed through the member→`branch_groups.branchName` integration step while `autoMerge` is off; this is a soft pre-integration only and does not permit shared-branch → default-branch promotion. RECONCILE-ONLY sweeps (branch rebind, blocker fan-out, stale-status clears, contamination metadata cleanup, attribution restore, PR refresh, misclassified-failure error clearing) continue to run.
+- **In-review is terminal-until-merged under `autoMerge: false` (FN-5147)**: when a project sets `settings.autoMerge: false`, `in-review` is the intended resting state until a human merges the PR. No lifecycle-mutating self-healing sweep (`reclaimSelfOwnedBranchConflicts`, `recoverGhostReviewTasks`, `recoverStaleIncompleteReviewTasks`, `recoverInterruptedMergingTasks`, `recoverStuckMergeDeadlocks`, `recoverMissingWorktreeReviewFailures`, `recoverPartialProgressNoTaskDoneFailures`, `recoverCompletionHandoffLimbo`, `recoverPostDoneNonContinuableWedge`, `recoverMergeableReviewTasks`, `recoverMergedReviewTasks`, `recoverAlreadyMergedReviewTasks`, `recoverOrphanOnlyScopeViolations`, `recoverForeignOnlyContaminatedInReviewTasks`, `recoverReviewTasksWithFailedPreMergeSteps`, `finalizeNoOpReviewTasks`, `surfaceInReviewStalls`, `surfaceInReviewStalled`) may move the task out of `in-review`, mark it `paused`/`failed`, or re-enqueue it for execution. Scoped FN-5819 exception: shared-group members (`branchContext.assignmentMode === "shared"`) are still allowed through the member→`branch_groups.branchName` integration step while `autoMerge` is off; this is a soft pre-integration only and does not permit shared-branch → default-branch promotion. RECONCILE-ONLY sweeps (branch rebind, blocker fan-out, stale-status clears, contamination metadata cleanup, attribution restore, PR refresh, misclassified-failure error clearing) continue to run.
 - **Auto-merge integration-root default (FN-5279)**: direct auto-merge now defaults `mergeIntegrationWorktree` to `reuse-task-worktree`; merger must pass the reuse handoff gates or emit `merge:reuse-handoff-refused` and leave the task in `in-review` without silently falling back to `cwd-integration-branch` (`cwd-main` remains a deprecated alias normalized to that mode).
 - **Orphaned execution sweep is observation-only (FN-5337)**: `recoverOrphanedExecutions` only annotates stale in-progress candidates with `task:orphan-detected-no-action` and `[orphan-detected] ... no action (operator-decides)` logs. It must never move `in-progress`/`in-review` backward to `todo` or mutate lease/worktree metadata. Proof-based backward recovery remains exclusively in `recoverInProgressLimbo` (FN-5219), `RestartRecoveryCoordinator`, `recoverMissingWorktreeReviewFailures`, and explicit executor/merger failure paths. Reintroducing lifecycle mutation here requires hard git/session proof gating plus CEO+CTO+PM sign-off.
 - **Self-owned reclaim resume-limbo escalation (FN-5704)**: `reclaimSelfOwnedBranchConflicts` tracks `resumeLimboCount`, `resumeLimboTipSha`, and `resumeLimboStepSignature` for in-progress reclaim/unpause loops. If reclaim finds no progress (same tip, same step-status signature, and no active-session signal) for `MAX_NO_PROGRESS_RESUME_ATTEMPTS` consecutive sweeps, self-healing escalates by moving the task to `todo` with `preserveWorktree: true`, `preserveProgress: true`, and `preserveResumeState: true` instead of endlessly re-arming resume. Escalation emits `task:resume-limbo-escalated` run-audit metadata (`frozenTipSha`, `idleMs`, `resumeAttemptCount`, `currentStep`) and resets the limbo counter.
@@ -1763,6 +1763,7 @@ Reliability-layer changes are in scope. Interaction regression backstops live in
 - FN-4976 backstop: `packages/engine/src/__tests__/reliability-interactions/stale-self-owned-session-registry.test.ts` guards `cleanupConflictingWorktree` clearing stale same-task `activeSessionRegistry` entries before the FN-4811 foreign-owner check, while preserving refusal behavior for foreign owners and live same-task bindings.
 - FN-5346 backstop: `packages/engine/src/__tests__/reliability-interactions/post-completion-stale-self-owned-binding.test.ts` covers post-completion and dep-abort same-task stale-binding cleanup, restart-residue recovery, same-task live-binding refusal, foreign-owner FN-4811 refusal, idempotent repeat sweeps, and FN-4954 lease-map composition.
 - FN-4999 backstop: `packages/engine/src/__tests__/reliability-interactions/completion-handoff-limbo.test.ts` covers the `recoverCompletionHandoffLimbo` sweep stage (grace window, active-task skip, merge-blocker guard, capped retries, and audit fan-out).
+- FN-5889 backstop: `packages/engine/src/__tests__/reliability-interactions/post-done-continuation-no-wedge.test.ts` covers the post-done step-session non-continuable suppression path plus `recoverPostDoneNonContinuableWedge`, including bounded self-heal ordering before stall surfacing.
 - FN-5345/FN-5377 backstops: `packages/engine/src/__tests__/reliability-interactions/merge-reuse-task-worktree.test.ts` (`FN-5345: empty-own-diff branch auto-finalizes via early fast-path without acquiring reuse handoff`) covers the early no-op fast-path under drifted worktree mapping; `packages/engine/src/__tests__/real-git/prepare-commit-msg-empty-guard.real-git.test.ts` covers the empty-commit refusal hook (refuses `--allow-empty`, allows amend + real commits, no-op outside fusion worktrees).
 - FN-5083 backstop: `packages/engine/src/__tests__/reliability-interactions/in-review-branch-rebind.test.ts` covers in-review branch rebind composition with metadata-cleared state, idempotent re-sweeps, and ambiguous-candidate skip behavior.
 - FN-5093 backstop: `packages/engine/src/__tests__/reliability-interactions/in-review-stalled-detector.test.ts` covers composition between quiet-window in-review stalled surfacing and adjacent reason-driven/paused/ghost-recovery/auto-merge gating paths.