The /title command would fail with 'Session not found in database.' when
used as the first command in a new session. This happened because:
1. Gateway creates session in session_store (in-memory)
2. But SQLite _session_db only gets sessions when agent flushes messages
3. set_session_title() does UPDATE which fails if row doesn't exist
Now we check if session exists in SQLite and create it if needed before
attempting to set the title.
Fixes: Session not found in database. error on /title in new chats
d3659c8ca0