feat(FN-4019): prevent merge-confirmed finalization loops via self-healing

Added self-healing logic to prevent merge-confirmed tasks from re-entering the executor, with test coverage for both merge error recovery and self-healing behavior, plus corresponding architecture and task-management docs.

Fusion-Task-Id: FN-4019

docs/architecture.md

@@ -598,7 +598,7 @@ Runtime action-gate flow (v1):
   - `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`.
   - `recoverMissingWorktreeReviewFailures()` is a narrow failed-review recovery: only `status: "failed"` `in-review` tasks with the explicit session-start signature `Refusing to start coding agent in missing worktree:` (from `assertValidWorktreeSession()`) are requeued. Recovery clears stale session metadata (`worktree`, `branch`, `sessionFile`, transient failure state), preserves valid step progress/retry counters, logs the auto-recovery reason, and moves the task back to `todo` for a clean retry.
   - `recoverMergeableReviewTasks()` only re-enqueues truly eligible tasks; retry-exhausted review tasks are skipped to avoid re-enqueue/no-op loops that keep refreshing `updatedAt`.
-  - `recoverAlreadyMergedReviewTasks()` auto-finalizes retry-exhausted `in-review` tasks when self-healing can prove their work already landed on the merge target, preventing deadlocked cards from remaining indefinitely in failed review state.
+  - `recoverAlreadyMergedReviewTasks()` auto-finalizes retry-exhausted `in-review` tasks when self-healing can prove their work already landed on the merge target, but it still honors `getTaskMergeBlocker()` before `in-review` → `done`. If a blocker remains (for example, incomplete steps), the task is surfaced in stable `in-review/failed` state with a blocker error instead of entering an auto-finalize loop.
   - No-`fn_task_done` recovery classification is normalized across executor, restart recovery, and self-healing: detection keys on executor-emitted `"without calling fn_task_done"` strings (while still tolerating legacy `task_done` wording), then applies the bounded ladder deterministically (in-session retries → bounded todo requeues with preserved progress when appropriate → terminal surfaced failure when budget is exhausted).
   - `clearStaleBlockedBy()` clears `blockedBy` (and transient `status`) on todo tasks when their blocker is missing, done, archived, paused in-review, or failed in-review with merge retries exhausted. FN-3924 extends this with a dependency-integrity guard: if a task has explicit dependencies and `blockedBy` is not one of the currently unresolved deps, the stale marker is cleared. This repairs rows corrupted by historical overlap re-stamping and lets scheduler re-evaluate from live dependency state.
   - Together, `recoverAlreadyMergedReviewTasks()`, `clearStaleBlockedBy()`, and paused-aware in-review scheduling prevent merge-deadlock loops by finalizing already-landed work, clearing stale dependency blockers, and avoiding paused review cards re-blocking overlap dispatch.

docs/task-management.md

@@ -93,6 +93,7 @@ Fusion task columns:
 4. **in-review** — implementation complete; awaiting finalization
    - If merge/finalization hits a terminal error, tasks can remain in `in-review` with `status: "failed"` for explicit follow-up. This state is intentionally preserved by recovery (not auto-bounced to `todo`).
    - Retry behavior splits by step completion: `in-review` tasks with incomplete steps (`pending`/`in-progress`) are treated as execution failures and retried back to `todo` with `preserveProgress: true`; `in-review` tasks with all steps `done` are treated as merge/finalization failures and stay in `in-review` with merge retry state reset.
+   - Merge-confirmed tasks still respect `getTaskMergeBlocker()` before the final `in-review` → `done` move. If merge is confirmed but a blocker remains (for example, incomplete steps), Fusion parks the task in `in-review` with `status: "failed"` and an explicit blocker error instead of retry-looping auto-finalization.
    - Self-healing can still auto-finalize retry-exhausted failed review tasks when it can prove their branch content already landed on the merge target, so already-merged work does not deadlock in `in-review`.
    - Non-recoverable state-machine errors during finalization (for example `Invalid transition: 'todo' → 'done'`) are treated as terminal review failures: recovery must not re-enqueue these tasks for merge unless task state changes prove they are recoverable.
 5. **done** — merged/finalized