feat(sentry): a quiet fire collapses to a one-line heartbeat

A sentry fire whose passes all probe-skip or no-op now writes a single "sentry at HH:MM: nothing" line instead of a full per-pass digest block. A fire that runs or queues anything writes the full digest as before. This is Phase 1 of the silent-until-signal policy. The live trial's fires 3-8 were walls of no-op digests, the noise this removes.

The "no silent skip" discipline is reconciled, not broken: inside a working fire every pass still writes its ran/skipped line, and a quiet fire's heartbeat is the explicit "nothing to do" record rather than a hidden skip.

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