feat: Add callbacks command for handling agent messages (gt-eph-5c8)

Implements `gt callbacks process` for Mayor/Deacon patrol to handle
callbacks from agents:

- POLECAT_DONE: Log polecat completion
- MERGE_COMPLETED: Close source issue, log merge
- MERGE_REJECTED: Log rejection reason
- HELP/ESCALATION: Forward to overseer
- SLING_REQUEST: Log sling request for spawn
- WITNESS_REPORT: Log rig health status
- REFINERY_REPORT: Log queue status

Also adds EventCallback to townlog for callback processing events.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/townlog/logger.go

@@ -27,6 +27,8 @@ const (
 	EventCrash EventType = "crash"
 	// EventKill indicates an agent was killed intentionally.
 	EventKill EventType = "kill"
+	// EventCallback indicates a callback was processed during patrol.
+	EventCallback EventType = "callback"
 )
 
 // Event represents a single agent lifecycle event.
@@ -143,6 +145,12 @@ func formatLogLine(e Event) string {
 		} else {
 			detail = "killed"
 		}
+	case EventCallback:
+		if e.Context != "" {
+			detail = fmt.Sprintf("callback: %s", e.Context)
+		} else {
+			detail = "callback processed"
+		}
 	default:
 		detail = string(e.Type)
 		if e.Context != "" {