swarm: cancel an agent's turn from the swarm queue

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

hive-agent/src/main.rs

@@ -566,7 +566,7 @@ async fn serve_main<S: Surface>(socket: &Path, poll_ms: u64) -> Result<()> {
     // Messages made inside this container rather than by the broker:
     // reminders and the swarm UI's operator input.
     let (local_msg_tx, local_msg_rx) = tokio::sync::mpsc::unbounded_channel();
-    swarm_input::spawn(local_msg_tx.clone());
+    swarm_input::spawn(local_msg_tx.clone(), bus.clone(), interrupted.clone());
     let reminder_store = match reminders::Reminders::open(&paths::state_db()) {
         Ok(store) => Some(Arc::new(store)),
         Err(e) => {