fix(issue-18): admit remote attempts atomically, end timed-out commands with their group, fence heartbeats off the terminal, renew liveness only on news

Review round 2 (planner-reviewer-codex-01, NOT PASS at d09c162):

- R1 remote.mirror_job: admission under the registry lock. The same
  dispatch_token is idempotent (a finished attempt never runs again), a
  lower attempt or the same attempt under another token is refused before
  any write, and only a strictly newer attempt replaces the record
  (register_job(supersede_older_attempt=True); events.log kept, seqs stay
  monotonic).
- R2 remote.handle_dispatch: the --exec command runs in its own session;
  at timeout_sec, or when the runner is interrupted (SIGTERM now unwinds
  it), the whole process group is terminated (TERM, then KILL) before the
  error is published.
- R3 terminal barrier: a terminal publish raises an attempt-scoped barrier
  under the lock that allocates seqs, stops the heartbeat timer, gives it
  MAM_HEARTBEAT_DRAIN_S (default 1 s) to exit and then kills it, and only
  then takes its own seq. Beats are admitted under that lock
  (next_seq(admit=...)); a beat still on the wire is cancelled with the
  timer rather than holding the terminal back; and a late non-terminal
  event never publishes a running state over a terminal record.
- R4 job_subscriber: a retained state renews the idle clock only when its
  seq is newer than everything already accepted; re-delivered or older
  snapshots still recover a terminal status.
- tests: five regressions, all failing on 77f2829; T1-14's wall clock
  15 -> 30 s against public-broker latency.
- docs: MESSAGING 2.5 (barrier, state renewal, detection vs final close),
  heartbeat docstring, loop SKILL.md (remote rows need the Python runner),
  VERSIONS v4.5.0 items 1/4/5 and the test count (25).

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

.agents/skills/lib_py/delegation/mqtt_common.py

@@ -515,12 +515,25 @@ def _update_job_status_legacy(job_id: str, registry_dir: str = DEFAULT_REGISTRY_
     return record
 
 
-def next_seq(job_id: str, registry_dir: str = DEFAULT_REGISTRY_DIR) -> int:
+class AdmissionRefused(Exception):
+    """A publish was refused under the registry lock before it took a seq."""
+
+
+def next_seq(job_id: str, registry_dir: str = DEFAULT_REGISTRY_DIR, admit=None) -> int:
     """Return the next monotonic sequence number for a job, persisted in the
     record's ``last_seq`` field so it stays consistent across process restarts.
-    First call returns 1."""
+    First call returns 1.
+
+    ``admit`` (optional) is called with the record under the same lock; a
+    non-empty return refuses the seq (``AdmissionRefused``) and writes nothing.
+    Heartbeat beats use it so that a terminal barrier and the seq allocation
+    are one atomic decision (review round 2, codex R3)."""
     with registry_lock(registry_dir):
         record = load_job(job_id, registry_dir)
+        if admit is not None:
+            reason = admit(record)
+            if reason:
+                raise AdmissionRefused(reason)
         seq = int(record.get("last_seq", 0)) + 1
         record["last_seq"] = seq
         record["updated_at"] = _utcnow()