fix: task_state reactivity + session_id in agent messages

- Auto-transition task_state from 'idle' to 'working' when agents start:
  - Base adapter: on set_summary call (agent declares what it's doing)
  - Monitor: on first Claude assistant output, tool_use, or Codex item.completed
  - Prevents dashboard showing 'idle' while agents are actively working

- Add session_id + from_slot_id to send_message in base adapter.
  Without session_id, agent-to-agent messages were stored with NULL
  session_id and excluded from dashboard message log queries.