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-loop/SKILL.md

@@ -36,6 +36,27 @@ Run an autonomous planning-execution-review loop using multiple agents (Planner,
 
 ---
 
+
+## Liveness contract and remote sessions (Issue #18)
+
+Every delegate job the loop submits carries the liveness contract of
+`multi-agent-mux-delegate-job`: agents heartbeat automatically once they
+publish `started --heartbeat N`, a job with no event for K x N seconds is
+closed as `deadline_timeout`, and `started` must arrive within T seconds of
+the injection. Tune it for a whole loop with environment variables, which the
+runner passes through as `--heartbeat` / `--heartbeat-misses` / `--ack-timeout`:
+
+```bash
+MAM_HEARTBEAT_SEC=30 MAM_HEARTBEAT_MISSES=4 MAM_ACK_TIMEOUT_S=60 \
+  bash .agents/skills/multi-agent-mux-loop/scripts/run_loop.sh --creator creator-claude-01 --all-reviewer --task "..."
+```
+
+A session row in `.mam/agent-sessions.yaml` with `transport: remote` is
+delegated as `remote:<name>` (the brief goes over the broker to a
+`remote.py run --session <name>` runner on another host; no herdr, pane or
+process probing happens for it). The row still needs `role` and
+`status: running` to be picked up by `--all-reviewer`.
+
 ## Roles & Responsibilities
 
 협업 시스템은 각 에이전트의 책임 영역을 명확히 격리하여 상호 교차 검증을 강제합니다.