Bind a sticky agent identity to this MCP session. Once bound, wake_status and load_context default to this agent_id when called without an explicit agent_id= arg — no need to pass identity on every call. Per-call override still works.

Persisted to mcp_sessions.agent_id (migration 0008), so the binding survives setup_account(mode='restore') and the server re-hydrates it automatically on the next tool call.

Call once near the start of a session, ideally from your boot recipe (HANDLER: bind_agent agent_id=). Idempotent: re-binding the same agent_id is a no-op. Pass agent_id='' to clear the binding.