A separate $SWARM.control.<agent> subject carries commands, apart from
the text subject. swarm-controller exposes POST /api/agents/{name}/cancel
as a NATS request; the harness answers whether a turn was actually
stopped, through the same cancel path as the dashboard button. The agent
credential gets subscribe on its control subject and allow_responses to
answer; only the controller may publish there.
Refs #3803
