- Add nexus_alert() function to send alerts to Nexus Watchdog
- Alerts are written as JSON files to $NEXUS_ALERT_DIR (default: /tmp/nexus-alerts)
- Alert includes: alert_id, timestamp, source, host, alert_type, severity, message, data
- Send 'stale_lock_reclaimed' warning alert when stale lock detected (age > 600s)
- Send 'heartbeat_resumed' info alert after successful recovery
- Include lock age, lockfile path, action taken, and stat info in alert data
- Add configurable NEXUS_ALERT_DIR and NEXUS_ALERT_ENABLED settings
- Add test script for validating alert functionality
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