* refactor(coding-agent): derive scoped heartbeats at render time * fix(coding-agent): keep the earliest heartbeat manager refresh deadline scheduleHeartbeatManagerRefresh now runs on every subagent snapshot update, and each call cleared and re-armed the refresh timer. An overdue heartbeat uses a relative 5s fallback delay, so children updating more often than every 5s postponed the open manager's refresh indefinitely. Track the scheduled deadline and keep the pending timer unless the newly derived deadline is earlier. * test(coding-agent): cover re-arming the heartbeat refresh to an earlier deadline The keep-timer-always mutant (returning whenever a timer is armed) survived the touched suites: nothing proved a sooner heartbeat pulls the pending refresh forward past the capped 60s poll.