fix: stop intercepting non-command : prefixed messages

Change handleExtensionCommand default case to pass through unrecognized
: prefixed messages as normal chat. Previously :) :D 🤔 etc.
would return "Unknown command" across all channels (Telegram, Discord).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -1292,7 +1292,9 @@ Token usage (this session):
 		), true
 
 	default:
-		return fmt.Sprintf("Unknown command: %s\nType :help for available commands.", cmd), true
+		// Don't intercept unrecognized : prefixed messages (e.g. :) :D :thinking:)
+		// Let them pass through as normal chat messages
+		return "", false
 	}
 }