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/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