feat(queue): add queue system with background daemon
Implements #134 - Queue system with background daemon.
## Changes
### Configuration
- QUEUE_DIR, QUEUE_ITEMS_DIR for queue storage
- QUEUE_DAEMON_PID_FILE, LOCK_FILE, LOG_FILE for daemon management
- QUEUE_DAEMON_INTERVAL_MINUTES (default: 5)
- QUEUE_DAEMON_BATCH_SIZE (default: 2)
- QUEUE_CLEANUP_AGE_DAYS (default: 7)
### Queue System
- File-based queue at ~/.kugetsu/queue/items/
- One JSON file per queue item
- States: pending, notified, completed, error
### New Commands
- kugetsu queue [list|stats|clear] - View queue status
- kugetsu queue enqueue <issue-ref> <message> - Manually enqueue
- kugetsu queue-daemon [start|stop|restart|status|logs] - Daemon management
### Behavior Change
- kugetsu delegate now always enqueues (fire-and-forget)
- Queue daemon polls queue and invokes PM when slots available
### Queue Item Format
```json
{
"id": "q_xxx",
"issue_ref": "github.com/user/repo#123",
"message": "task description",
"state": "pending",
"pending_since": "...",
"notified_at": null,
"completed_at": null,
"error": null
}
```
Closes #134
043542344a