fix(teardown): refuse to release a worktree another task still records

[karanmrn]

Why

On 1 Oct 2026, tearing down a stale task (pubmax-map-ui-sweep, whose work had moved elsewhere) returned treehouse slot pubmaxx-bde241/19 to the pool while another live task, pubmax-out-honesty-guard, was working in it. That reset the live worker's copy to detached main. Its commits survived, but nothing stopped the release.

Change

• bin/fm-teardown.sh: before releasing a worktree, look for any other state/<id>.meta that records the same worktree path. If that task's endpoint is alive, or it records a different branch, teardown refuses to return, reset, or clean the slot and names the conflicting task. The rest of the stale task's cleanup (its own records, endpoint, hooks it owns) still completes. A dead record on the same branch does not block release, and it never weakens the existing unlanded-work refusal.
• bin/fm-backend.sh: small helper changes the guard uses to read neighbor endpoint liveness.
• tests/fm-teardown.test.sh: regression cases for the shared-slot guard.

Tests

Reproduced first: with two metas pointing at the same worktree, tearing down the stale one returned the slot and cleaned the neighbor's hook files. After the fix:

• bash tests/fm-teardown.test.sh: exit 0, 65 ok, 0 not ok. New cases:
  • a live neighbor recording the same worktree blocks slot return, reset, and cleanup
  • a neighbor on a different branch blocks slot return even when its endpoint is dead
  • a dead record on this task's own branch does not block slot return
  • another task's unpushed branch does not block stale-record cleanup or get returned
  • a dead same-branch neighbor does not weaken the unlanded-work refusal
• bin/fm-lint.sh: exit 0 (ShellCheck 0.11.0, actionlint 1.7.12, pinned).

Validation path

This PR skips the no-mistakes pipeline by the captain's decision on 2 Oct 2026, because no-mistakes refuses every available gate agent on the firstmate repo. It ships as a direct PR; GitHub CI is the gate.

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