feat(gateway): Enable Slack thread replies without explicit @mentions

When a user replies in a Slack thread where the bot has an active
conversation session, the bot now processes the message even without
an explicit @mention. This improves UX for ongoing threaded
discussions.

Changes:
- Added set_session_store() to BasePlatformAdapter for adapters to
  check active sessions
- Modified SlackAdapter to detect thread replies and check if a
  session exists for that thread before requiring @mentions
- Updated GatewayRunner to inject the session store into adapters
- Added comprehensive tests for the new behavior

Fixes: Thread replies without @jarvis are now processed if there is
an active session, matching user expectations for conversation flow

gateway/platforms/base.py

@@ -569,6 +569,16 @@ class BasePlatformAdapter(ABC):
         """
         self._message_handler = handler
     
+    def set_session_store(self, session_store: Any) -> None:
+        """
+        Set the session store for checking active sessions.
+        
+        Used by adapters that need to check if a thread/conversation
+        has an active session before processing messages (e.g., Slack
+        thread replies without explicit mentions).
+        """
+        self._session_store = session_store
+    
     @abstractmethod
     async def connect(self) -> bool:
         """