feat: implement channel directory and message mirroring for cross-platform communication

- Introduced a new channel directory to cache reachable channels/contacts for messaging platforms, enhancing the send_message tool's ability to resolve human-friendly names to numeric IDs.
- Added functionality to mirror sent messages into the target's session transcript, providing context for cross-platform message delivery.
- Updated the send_message tool to support listing available targets and improved error handling for channel resolution.
- Enhanced the gateway to build and refresh the channel directory during startup and at regular intervals, ensuring up-to-date channel information.

gateway/session.py

@@ -367,6 +367,11 @@ class SessionStore:
         
         return False
     
+    def has_any_sessions(self) -> bool:
+        """Check if any sessions have ever been created (across all platforms)."""
+        self._load()
+        return len(self._entries) > 1  # >1 because the current new session is already in _entries
+    
     def get_or_create_session(
         self, 
         source: SessionSource,