fix(FN-5256): keep live task worktrees through pause/resume races

Three independent reliability fixes that all surfaced as the same bug:
live tasks losing their worktrees mid-execution and emitting
`wrong_toplevel` errors.

Fix A — executor stale-self-owned classifier:
`reconcileSelfOwnedActiveSessionForRemoval` now takes a process-active
probe (`executingTaskLock.has`) and a minimum-idle window (default 5s)
in addition to the existing in-memory `activeWorktrees` binding probe.
Recently-registered or still-running entries are refused with
`process-active-refuses` / `too-recent-refuses`, with audit-grade
log lines. Both the pre-remove path
(`reconcileSelfOwnedBeforeRemove`), the post-throw retry in
`removeOwnWorktreeWithReconcile`, and the defensive reconcile in
`removeWorktree` route through the same hardened gates.

Fix B — pause synchronously reaps the agent session:
New `awaitAbortInFlightTaskWork` mirrors the existing fire-and-forget
abort but awaits each `session.abort()` /
`stepExecutor.terminateAllSessions()` /
`workflowSession.abort()`. `parkTaskAfterWorkflowStepPause` calls it
before `moveTask("todo")`, and the `task:updated` user-pause handler
routes through it, so a fast re-dispatch can no longer race a still-
live shell.

Fix C — self-healing realpath + active-task skip:
`reconcileTaskWorktreeMetadata` now realpath-normalizes both sides of
the registry comparison (handling macOS `/private/var/...`) and
refuses to clear `worktree`/`branch` on in-progress or in-review
tasks. The skip emits a new
`task:auto-recover-worktree-metadata-skipped-active` audit event;
executor-level recovery paths remain in charge of active tasks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/worktree-backend.test.ts

@@ -873,6 +873,9 @@ describe("removeWorktree", () => {
       reason: RemovalReason.ExecutorDispose,
       expectedOwnerTaskId: "FN-1",
       liveOwnerProbe: () => false,
+      // FN-5256: opt out of the min-idle window so this defensive-reconcile test
+      // is unaffected by the new warm-up gate.
+      reconcileMinIdleMs: 0,
     });
 
     expect(activeSessionRegistry.lookupByPath("/repo/.worktrees/fn-1")).toBeNull();