fix(issue-18): let an exiting heartbeat timer skip the busy lifecycle lock; kill a timer whose pid cannot be recorded

Review round 5 (planner-reviewer-codex-01, NOT PASS at 2b1d590):

- F1 bounded lock cycle: the stopper holds the per-job lifecycle lock while
  it waits for the timer to exit, and the timer's exit cleanup waited for
  that same lock, so every stop ran out the drain and ended in SIGKILL with
  a false "did not drain" warning. The exiting timer now tries the lock
  without blocking (lifecycle_lock(blocking=False)); when it is busy the
  ownership operation holding it removes the pidfile, and when it is free
  the own-pid comparison is kept.
- F2 untracked owner: when writing the new timer's pid fails after the
  spawn, _spawn_timer_locked kills and reaps that exact process while the
  lock is still held and drops only a partial pidfile of that attempt,
  then re-raises.
- tests: an idle timer stops well before a 10 s drain without SIGKILL;
  a pidfile failure after spawn leaves no survivor, no pidfile and a free
  lock (both fail on 2b1d590); a spawn failure leaves nothing behind and
  the next start works.
- docs: MESSAGING 2.5 and VERSIONS item 1 describe the lifecycle lock;
  VERSIONS test count 31.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

MESSAGING.md

@@ -196,7 +196,7 @@ orchestrator's disk, take part on equal terms.
 
 | Rule | Contract | Where |
 |---|---|---|
-| **Heartbeat obligation** | While working, the agent publishes `progress` at least every **N** s (`heartbeat_sec`, default 30). `publish_event.py --event started --heartbeat N` starts a detached timer that publishes `progress` (`detail: heartbeat`) every N s with the same `dispatch_token`/`attempt_id`, so the agent does not have to remember. No heartbeat can follow a terminal event: the terminal publish first raises an attempt-scoped barrier (`<registry>/<job>.heartbeat.barrier`, written under the registry lock that also allocates seqs, so a beat either took its seq before it or is refused), then stops the timer through `<registry>/<job>.heartbeat.pid` and waits for it to drain its in-flight beats (killing it after `MAM_HEARTBEAT_DRAIN_S`, default 1 s - a beat still on the wire is cancelled, never allowed to trail the terminal), and only then takes its own seq; replacing the timer (another `started --heartbeat` for the job) drains or kills the previous one the same way before it hands over the pidfile. The timer also stops itself on a terminal record or barrier, on re-dispatch, after `timeout_sec`, and within one period of the agent process it watches going away (`--watch-pid`, default the nearest agent-CLI ancestor of the publishing command; a remote runner watches itself) - a crashed agent stops heartbeating, an alive-but-stuck one does not. | `lib_py/delegation/heartbeat.py`, `publish_event.py --heartbeat` |
+| **Heartbeat obligation** | While working, the agent publishes `progress` at least every **N** s (`heartbeat_sec`, default 30). `publish_event.py --event started --heartbeat N` starts a detached timer that publishes `progress` (`detail: heartbeat`) every N s with the same `dispatch_token`/`attempt_id`, so the agent does not have to remember. No heartbeat can follow a terminal event: the terminal publish first raises an attempt-scoped barrier (`<registry>/<job>.heartbeat.barrier`, written under the registry lock that also allocates seqs, so a beat either took its seq before it or is refused), then stops the timer through `<registry>/<job>.heartbeat.pid` and waits for it to drain its in-flight beats (killing it after `MAM_HEARTBEAT_DRAIN_S`, default 1 s - a beat still on the wire is cancelled, never allowed to trail the terminal), and only then takes its own seq; replacing the timer (another `started --heartbeat` for the job) drains or kills the previous one the same way before it hands over the pidfile. Starts, replacements and stops of a job's timer are serialized by a per-job lifecycle lock (`<registry>/<job>.heartbeat.lock`, separate from the seq lock): a start that finds the attempt ended arms nothing, an exiting timer never waits for that lock, and a timer whose pid could not be recorded is killed at once. The timer also stops itself on a terminal record or barrier, on re-dispatch, after `timeout_sec`, and within one period of the agent process it watches going away (`--watch-pid`, default the nearest agent-CLI ancestor of the publishing command; a remote runner watches itself) - a crashed agent stops heartbeating, an alive-but-stuck one does not. | `lib_py/delegation/heartbeat.py`, `publish_event.py --heartbeat` |
 | **Unresponsive** | The orchestrator treats a job as unresponsive after **K x N** s without any event (`heartbeat_misses`, default 4 → 120 s; `--idle-timeout` overrides). K x N is when silence is *detected*; the wrapper's post-idle grace below can stretch the final close to about 2 x K x N. The subscriber's idle clock counts every accepted event, and a retained state only when its seq is newer than every event and state already accepted - the same snapshot re-delivered on a reconnect, or an older one, is not liveness; the wrapper's post-idle grace re-reads only the job's events.log (`_recent_disk_heartbeat`, written before every wire publish). herdr status and the pane are logged once as a *diagnostic* (`liveness: ... herdr/pane says busy=yes/no (diagnostic only)`) and never decide. | `multi-agent-mux-delegate-job` (`finalize_unfinished`), `job_subscriber.py` |
 | **Delivery** | After the prompt is handed over (herdr accepted the submission, or the remote dispatch was published), the agent's `started` must arrive within **T** s (`ack_timeout_sec`, default 180; a real agent reads the brief before it runs the start command): the registry leaves `pending`, a `started` line lands in events.log, or the subscriber prints one. No `started` within T → the inject fails closed: rc 1, registry `error` (`agent_blocked`), and the prompt is **never re-injected**. `send_keys_safe` confirms an accepted-but-unconfirmed submission only by `agent_status == working` or the job's `started`; pane changes and busy tokens are no longer signals. | `_wait_started`, `lib.sh: _sks_job_started` |
 | **Termination** | Only `completed` / `error` (and an operator `cancelled`) end a job. Terminal immutability (Issue #6 CAS) refuses anything that arrives after `deadline_timeout` closed the job. | `engine.py` |