feat(FN-3498): add self-healing and ownership-aware done-task merge reconci

The merge completes FN-3498 across three steps: adds ownership-aware done-task reconciliation to the merger, prevents branch-missing head SHA pollution during merge operations, and restores workspace typecheck compatibility. Core changes touch the merger (103 lines) and self-healing module (67 lines

Fusion-Task-Id: FN-3498

docs/architecture.md

@@ -405,6 +405,7 @@ See [Memory Plugin Contract](./memory-plugin-contract.md) for the full plan.
 - `SelfHealingManager` (`self-healing.ts`) — auto-unpause/maintenance recovery actions
   - `recoverGhostReviewTasks()` is a fallback only for idle, non-terminal `in-review` states. Terminal/actionable states (notably `status: "failed"`) are preserved and **not** auto-kicked back to `todo`.
   - `recoverMergeableReviewTasks()` only re-enqueues truly eligible tasks; retry-exhausted review tasks are skipped to avoid re-enqueue/no-op loops that keep refreshing `updatedAt`.
+  - Merge commit attribution is ownership-aware: a `mergeDetails.commitSha` is trusted only when reachable from `HEAD` **and** attributable to the task via `Fusion-Task-Id` trailer or task-ID-bearing subject. Reachable-but-unowned SHAs are rejected to prevent sibling done tasks from sharing misleading merge metadata.
 - `ProjectEngine` settings lifecycle handlers (`project-engine.ts`) treat `enginePaused` as a soft pause: clearing it dispatches runtime resume and, when `autoMerge` is enabled, performs an `in-review` eligibility sweep to requeue mergeable review tasks.
 - `UsageLimitPauser` (`usage-limit-detector.ts`) and `withRateLimitRetry` (`rate-limit-retry.ts`)