feat(cron): add daemon_alive() two-level heartbeat check

[yubingz]

Summary

Implements a two-level daemon liveness check for the cron subsystem:

1. PID check: Verify PID file exists and process is actually running
2. Heartbeat check: Verify heartbeat file was updated within the configured interval

Returns True only if both checks pass. Falls back gracefully when either file is missing.

Also includes the complete daemon lifecycle management:

• _daemonize(): Fork to background with proper PID file handling
• run_ticker(): Periodic execution loop with heartbeat updates
• stop_daemon(): Clean shutdown via SIGTERM
• Signal handling (SIGTERM/SIGINT) + atexit cleanup

Closes #69976

[alt-glitch]

This was generated by AI during triage.

Related to #43864 and #69976. This standalone cron.daemon design calls the scheduler directly, while #43864 wires daemon mode through the CLI; these are competing contracts rather than duplicates.

[teknium1]

Thanks for addressing cron liveness. The heartbeat work on current main has moved into the provider lifecycle, so this needs substantial re-scoping before it can be safely salvaged.

Problems

• cron/daemon.py:64 catches PermissionError via OSError, deletes the PID file, and reports the daemon dead. os.kill(pid, 0) raising PermissionError indicates that the process exists but is not signalable; the linked issue explicitly calls for this case to return true.
• cron/daemon.py:118 calls cron.scheduler.tick() directly. Current execution resolves and starts the configured scheduler provider in gateway/run.py:25491-25542; cron/scheduler_provider.py:122-159 supports external providers, while the built-in provider owns heartbeat and profile-aware lifecycle behavior.
• The PR contains no tests for the specified PID/heartbeat cases.

Suggested changes

• Resolve the competing standalone-daemon contract with #43864, then integrate through the selected hermes cron / provider lifecycle.
• Preserve a live PID on PermissionError and add the listed liveness and integration tests.

Automated hermes-sweeper review.

[teknium1]

PermissionError is an OSError, but for os.kill(pid, 0) it means the PID exists and is not signalable by this user. Do not unlink the PID file or return false for that case; handle it separately as a live daemon.

[teknium1]

This bypasses the current CronScheduler provider lifecycle. Please resolve the configured provider and use its start/stop contract so external providers, profile scoping, and the built-in heartbeat behavior are not silently skipped.