Send a heartbeat to maintain presence and update activity.
Returns: Updated presence entry.
Raises: NotFoundError: 404 if agent is not registered.
Successful Response
Presence registration for an active agent or human session.
Attributes: agent_id: Unique identifier for the connected agent or session. agent_type: Whether the agent is external (MCP), internal, or human. name: Human-readable display name. organization_id: Organization the agent belongs to. activity: Description of current activity, if any. resource_id: Resource the agent is currently working on, if any. registered_at: Timestamp when presence was first registered. last_heartbeat: Timestamp of the most recent heartbeat.