- Add HeartbeatMetrics struct with uptime, consecutive success/failure
counts, EMA tick duration, and total ticks
- Add compute_adaptive_interval() for exponential backoff on failures
and faster polling when high-priority tasks are present
- Add SQLite-backed task run history (src/heartbeat/store.rs) mirroring
the cron/store.rs pattern with output truncation and pruning
- Add dead-man's switch that alerts if heartbeat stops ticking
- Wire metrics, history recording, and adaptive sleep into daemon worker
- Add config fields: adaptive, min/max_interval_minutes,
deadman_timeout_minutes, deadman_channel, deadman_to, max_run_history
- All new fields are backward-compatible with serde defaults
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