The Claude Code hook already receives a session_id in every payload, but
the activity route dropped it and the daemon had no field to store it on.
Parse it out of the hook body, add Session.agent_session_id, and write it
through to the sidecar once — the first report that carries one wins, and
later reports never overwrite it since an agent's own id never changes
mid-session.
This is the input to a follow-up POST /api/sessions/:id/resume.