feat: add /resume command to gateway for switching to named sessions

Messaging users can now switch back to previously-named sessions:
- /resume My Project  — resolves the title (with auto-lineage) and
  restores that session's conversation history
- /resume (no args)   — lists recent titled sessions to choose from

Adds SessionStore.switch_session() which ends the current session and
points the session entry at the target session ID so the old transcript
is loaded on the next message. Running agents are cleared on switch.

Completes the session naming feature from PR #720 for gateway users.

8 new tests covering: name resolution, lineage auto-latest, already-on-
session check, nonexistent names, agent cleanup, no-DB fallback, and
listing titled sessions.

gateway/session.py

@@ -593,7 +593,49 @@ class SessionStore:
                 logger.debug("Session DB operation failed: %s", e)
         
         return new_entry
-    
+
+    def switch_session(self, session_key: str, target_session_id: str) -> Optional[SessionEntry]:
+        """Switch a session key to point at an existing session ID.
+
+        Used by ``/resume`` to restore a previously-named session.
+        Ends the current session in SQLite (like reset), but instead of
+        generating a fresh session ID, re-uses ``target_session_id`` so the
+        old transcript is loaded on the next message.
+        """
+        self._ensure_loaded()
+
+        if session_key not in self._entries:
+            return None
+
+        old_entry = self._entries[session_key]
+
+        # Don't switch if already on that session
+        if old_entry.session_id == target_session_id:
+            return old_entry
+
+        # End the current session in SQLite
+        if self._db:
+            try:
+                self._db.end_session(old_entry.session_id, "session_switch")
+            except Exception as e:
+                logger.debug("Session DB end_session failed: %s", e)
+
+        now = datetime.now()
+        new_entry = SessionEntry(
+            session_key=session_key,
+            session_id=target_session_id,
+            created_at=now,
+            updated_at=now,
+            origin=old_entry.origin,
+            display_name=old_entry.display_name,
+            platform=old_entry.platform,
+            chat_type=old_entry.chat_type,
+        )
+
+        self._entries[session_key] = new_entry
+        self._save()
+        return new_entry
+
     def list_sessions(self, active_minutes: Optional[int] = None) -> List[SessionEntry]:
         """List all sessions, optionally filtered by activity."""
         self._ensure_loaded()