Implement replaceable notifications to prevent heartbeat stacking when
agents are busy. Only the latest notification per slot is delivered.
Changes:
- Add NotificationManager for tracking pending notifications
- Add SendKeysReplace() that clears input line before sending
- Integrate slot tracking into daemon heartbeat pokes
- Mark notifications consumed when agent shows activity
The system tracks pending notifications in state files and skips
sending if a notification for the same slot is still pending.
When agent activity is detected (keepalive), slots are marked
consumed allowing new notifications to be sent.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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