The router was calling handle_message(user_id, message, context, session_id)
but MasterAgent accepts (user_id, channel_id, message, directive_id) and
returns MasterResponse{response, status, ...} with no .reply or
.agent_reports fields. Discuss DMs to the bot crashed with TypeError.
Now the router:
- Derives directive_id from session_id (or generates one)
- Pulls channel_id out of req.context
- Maps MasterResponse.response -> DispatchResponse.reply
- Returns an empty agent_reports list (the field is reserved for future use;
per-agent reports aren't part of MasterResponse)
