Silently drop unknown bang commands instead of responding

Unknown !commands (like !mail, !login, !bbs from advBBS users) were
getting "Unknown command, try !help" responses. Now returns None so
the bot stays silent on commands it doesn't recognize.

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

meshai/commands/dispatcher.py

@@ -141,8 +141,7 @@ class CommandDispatcher:
         handler = self._commands.get(cmd)
 
         if handler is None:
-            # Unknown command
-            return f"Unknown command: !{cmd.lower()}. Try !help"
+            return None
 
         try:
             logger.debug(f"Dispatching !{cmd.lower()} from {context.sender_id}")