feat(FN-5624): suppress transient task.json ENOENT with guard, logging, and

Implements graceful suppression of transient `task.json` ENOENT errors in the executor, logging a suppression signal and surfacing a banner in the UI, with test coverage for both the executor behavior and notification service. Documentation in `docs/architecture.md` and a changeset for `@runfusion/f

Fusion-Task-Id: FN-5624

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5624

docs/architecture.md

@@ -685,6 +685,7 @@ Guardrails: this routine does **not** retry merges, does **not** apply to mixed/
   - FN-4285 decision: add a follow-up for a tree-equality recovery strategy (`rev-parse <base>^{tree}` == `<task-branch>^{tree}`) in `findAlreadyMergedTaskCommit`. This closes stranded already-merged branches that evade trailer/ancestry/patch-id matching, with guardrails limited to retry-exhausted review tasks to avoid false positives during transient post-rebase parity windows.
   - 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. FN-4091 broadens the sweep to active `in-progress` and un-paused `in-review` tasks as well, but those repairs only null `blockedBy` (they do not rewrite scheduler-owned queued state). FN-5488 adds two fast paths: (1) failed in-review blockers at/above `MAX_AUTO_MERGE_RETRIES` always fan out unblock recovery with explicit reason codes, and (2) `status="merging"|"merging-pr"` blockers with no active merger owner are treated as unbacked after a short grace window (`unbackedMergingFanoutGraceMs`, default 60s) so manual retry/unpause `updatedAt` refreshes cannot deadlock downstream todos indefinitely. Recovery logs now use `Auto-recovered (FN-5488): ... reason=<code>` for auditability while preserving FN-4538 overlap-blocking invariants.
+  - FN-5624 suppresses transient worktree-local `.fusion/tasks/<id>/task.json` ENOENT session-start failures. When the missing file path is under `task.worktree`, executor routes through unusable-worktree auto-recovery, skips persisting `status: "failed"`/`error` on the task row, and emits `[transient-task-json-suppressed] ... reason=missing-task-json-under-worktree`. The corresponding self-healing `Auto-recovered:` log entry keeps notification suppression aligned with the existing `/^Auto-recovered:/` grace-window rule.
   - `inspectBranchConflict()` now treats self-owned zero-attribution collisions as reclaimable (instead of foreign) when ownership is proven by task/worktree identity, so stranded self-branches do not enter unrecoverable loops.
   - `reclaimSelfOwnedBranchConflicts()` includes paused `branch-conflict-unrecoverable` tasks (not just todo/in-progress), clearing paused/error state in one update and requeueing only when parked in `in-review`.
   - Together, `recoverAlreadyMergedReviewTasks()`, `clearStaleBlockedBy()`, and paused-aware in-review scheduling prevent merge-deadlock loops by finalizing already-landed work, clearing stale dependency blockers, reclaiming self-owned conflicts, and avoiding paused review cards re-blocking overlap dispatch.