fix(gateway): Pass session_db to AIAgent, fixing session_search error

When running via the gateway (e.g. Telegram), the session_search tool
returned: {"error": "session_search must be handled by the agent loop"}

Root cause:
- gateway/run.py creates AIAgent without passing session_db=
- self._session_db is None in the agent instance
- The dispatch condition "elif function_name == 'session_search' and self._session_db"
  skips when _session_db is None, falling through to the generic error

This fix:
1. Initializes self._session_db in GatewayRunner.__init__()
2. Passes session_db to all AIAgent instantiations in gateway/run.py
3. Adds defensive fallback in run_agent.py to return a clear error when
   session_db is unavailable, instead of falling through

Fixes #105

run_agent.py

@@ -1467,14 +1467,17 @@ class AIAgent:
                 tool_duration = time.time() - tool_start_time
                 if self.quiet_mode:
                     print(f"  {_get_cute_tool_message_impl('todo', function_args, tool_duration, result=function_result)}")
-            elif function_name == "session_search" and self._session_db:
-                from tools.session_search_tool import session_search as _session_search
-                function_result = _session_search(
-                    query=function_args.get("query", ""),
-                    role_filter=function_args.get("role_filter"),
-                    limit=function_args.get("limit", 3),
-                    db=self._session_db,
-                )
+            elif function_name == "session_search":
+                if not self._session_db:
+                    function_result = json.dumps({"success": False, "error": "Session database not available."})
+                else:
+                    from tools.session_search_tool import session_search as _session_search
+                    function_result = _session_search(
+                        query=function_args.get("query", ""),
+                        role_filter=function_args.get("role_filter"),
+                        limit=function_args.get("limit", 3),
+                        db=self._session_db,
+                    )
                 tool_duration = time.time() - tool_start_time
                 if self.quiet_mode:
                     print(f"  {_get_cute_tool_message_impl('session_search', function_args, tool_duration, result=function_result)}")