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

tests/fixtures/issue_5/phase1/help-job_subscriber.txt

@@ -2,7 +2,7 @@ usage: job_subscriber.py [-h] (--job JOB | --wait-any) [--timeout TIMEOUT]
                          [--idle-timeout IDLE_TIMEOUT] [--expect-retention]
                          [--expect-dispatch-token EXPECT_DISPATCH_TOKEN]
                          [--attempt-id ATTEMPT_ID]
-                         [--registry-dir REGISTRY_DIR] [-v]
+                         [--registry-dir REGISTRY_DIR] [--remote] [-v]
 
 Subscribe to Job events on MQTT
 
@@ -21,4 +21,8 @@ optional arguments:
   --attempt-id ATTEMPT_ID
                         minimum attempt id to accept events for
   --registry-dir REGISTRY_DIR
+  --remote              the agent shares no disk with this registry: mirror
+                        started/terminal events into the job record (Issue
+                        #18; automatic for jobs registered with --transport
+                        remote)
   -v, --verbose

tests/fixtures/issue_5/phase1/help-publish_event.txt

@@ -6,7 +6,8 @@ usage: publish_event.py [-h] (--job JOB | --pick-pending)
                         [--finalize-report PATH]
                         [--side-effect KIND=REFERENCE]
                         [--attempt-id ATTEMPT_ID]
-                        [--expect-dispatch-token EXPECT_DISPATCH_TOKEN] [-v]
+                        [--expect-dispatch-token EXPECT_DISPATCH_TOKEN]
+                        [--heartbeat N] [--no-state] [-v]
 
 Publish a Job event to MQTT
 
@@ -33,4 +34,9 @@ optional arguments:
                         job's current iteration)
   --expect-dispatch-token EXPECT_DISPATCH_TOKEN
                         expected dispatch token for CAS verification
+  --heartbeat N         after publishing, start a detached timer that
+                        publishes a `progress` heartbeat every N seconds until
+                        the job ends (Issue #18; not with a terminal event)
+  --no-state            do not publish the retained <topic_prefix>/state
+                        message
   -v, --verbose