Support wildcard "*" in allowFrom channel config

Allow "*" in the allowFrom list to explicitly permit all senders,
as an alternative to the empty-list-means-allow-all behavior.

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

nanobot/channels/base.py

@@ -69,11 +69,15 @@ class BaseChannel(ABC):
             True if allowed, False otherwise.
         """
         allow_list = getattr(self.config, "allow_from", [])
-        
+
         # If no allow list, allow everyone
         if not allow_list:
             return True
-        
+
+        # Wildcard allows everyone
+        if "*" in allow_list:
+            return True
+
         sender_str = str(sender_id)
         if sender_str in allow_list:
             return True