feat(issue-18): decide liveness and delivery by the event protocol, not the screen

Contract (no new event types): heartbeat every N s (automatic timer),
unresponsive after K x N s without an event, `started` within T s of the
injection, retained per-job state, and a remote transport.

- registry: heartbeat_sec / heartbeat_misses / ack_timeout_sec / transport
  fields; idle_timeout_sec derives K x N unless explicit (defaults 30/4/60
  keep the historical 120 s idle window).
- heartbeat.py + publish_event --heartbeat N: detached fixed-cadence timer
  publishing signed `progress` heartbeats (threaded wire publish, local
  record first); stopped by the terminal event via pidfile, by re-dispatch
  and by lifetime. Terminal events refuse --heartbeat.
- publish_event: retained `<prefix>/state` (status, last_event, seq, HMAC)
  sent on the same connection after every event (--no-state to skip);
  publisher.publish_raw takes extra messages.
- job_subscriber: subscribes the state topic, recovers terminal status from
  retained state, resets idle on any accepted message, records seq_gap and
  reconnected in the audit log, idle budget before the first event is
  T + K x N (--ack-timeout), --remote mirrors started/terminal events into
  the registry through the transition boundary.
- lib.sh send_keys_safe: an accepted-but-unconfirmed submission is confirmed
  by agent_status working or the job's own `started` (_sks_job_started);
  pane change and busy token are no longer delivery signals.
- wrapper: --heartbeat/--heartbeat-misses/--ack-timeout (MAM_* env),
  injected start command arms the timer, brief states the contract,
  _wait_started (registry / events.log / subscriber log, bounded by T and
  the wall clock) replaces the 10 s inject grace, R1 grace uses only the
  on-disk event age (herdr/pane logged once as a diagnostic),
  remote:<name> sessions skip preflight/idle/pane and dispatch over the
  broker (remote_dispatch).
- remote.py (+ scripts shim): signed retained dispatch (MAM_DISPATCH_SECRET)
  and a runner that mirrors the job into its own registry with the
  orchestrator's identity, publishes started --heartbeat, runs --exec and
  publishes the terminal event.
- loop runner: passes MAM_HEARTBEAT_SEC / MAM_HEARTBEAT_MISSES /
  MAM_ACK_TIMEOUT_S through; YAML rows with transport: remote delegate as
  remote:<name>.
- docs: MESSAGING.md 2.5 contract + remote transport, delegate-job SKILL.md,
  registry.md, loop SKILL.md.
- tests: tests/test_issue18_liveness_contract.py (16); issue 14 pane-signal
  tests flipped to started-based; issue 6 working-agent tests acknowledge
  with started; CLI snapshot/help captures refreshed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

.agents/skills/multi-agent-mux-delegate-job/registry.md

@@ -193,3 +193,16 @@ $PY scripts/registry.py logs --list              # every job, live status
 ---
 
 > **구현 위치 각주 (v4.3.0)**: 본 문서가 인용하는 `scripts/*.py` 경로는 유효하며 계약도 불변이다. 실제 구현은 issue #5 phase 1에서 `.agents/skills/lib_py/delegation/`으로 이동했고, `scripts/*.py`는 그 패키지를 자기 옆 트리에 바인딩하는 호환 shim이다.
+
+## Liveness contract fields (Issue #18)
+
+| Field | Meaning | Default |
+|---|---|---|
+| `heartbeat_sec` | N: the agent publishes `progress` at least every N s (the `--heartbeat N` timer does it) | 30 |
+| `heartbeat_misses` | K: missed heartbeats before the job is unresponsive | 4 |
+| `idle_timeout_sec` | K x N unless `--idle-timeout` was given | 120 |
+| `ack_timeout_sec` | T: `started` must arrive within T s of the injection | 60 |
+| `transport` | `herdr` (pane injection) or `remote` (signed retained dispatch over the broker) | `herdr` |
+
+`<registry>/<job>.heartbeat.pid` / `.heartbeat.log` belong to the running heartbeat timer; a terminal publish removes the pidfile.
+