The first scheduler heartbeat (#95) only fired AFTER each tick completed.
A tick that runs fireSchedule for 110+ seconds (long agent prompts) would
make /admin/liveness report scheduler as stale even though it was actively
working. Observed today: scheduler firing UIUX audit, last_tick_at lagged
by 95s+ and incrementing.
Three places now call Heartbeat:
1. Top of tick() — proves we're past the ticker.C wait
2. Inside each fire goroutine, before fireSchedule — ANY active fire
keeps the heartbeat fresh
3. Inside each fire goroutine, after fireSchedule — captures the moment
the per-fire work completes
(The post-tick Heartbeat in Start() is still there as the "all idle" case.)
Net result: /admin/liveness reports stale only if the scheduler genuinely
isn't doing anything for >2× pollInterval, which is the actual signal we
want.
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