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/web_ui/mod.rs

@@ -339,6 +339,35 @@ fn find_claude_child() -> Option<u32> {
     None
 }
 
+/// Stop the turn in flight and say so in the live panel as `by`. Returns
+/// whether a turn was actually stopped: a cancel that raced an already
+/// finished turn is `false`, and only a `true` marks the next wake prompt as
+/// cut off. Shared by the dashboard's `/cancel` and the swarm queue's cancel
+/// command, so the two cannot disagree about what cancelling is.
+pub(crate) fn cancel_turn(bus: &Bus, interrupted: &AtomicBool, by: &str) -> bool {
+    let (acted, what) = if let Some(canceller) = bus.turn_canceller() {
+        if canceller.cancel() {
+            (true, "sent session/cancel to the ACP agent".to_owned())
+        } else {
+            (false, "no ACP turn to cancel".to_owned())
+        }
+    } else {
+        match sigint_claude() {
+            SigintOutcome::Signalled => (true, "sent SIGINT to claude".to_owned()),
+            SigintOutcome::NoProcess => (false, "no claude process to interrupt".to_owned()),
+            SigintOutcome::Failed(e) => (false, format!("kill failed: {e}")),
+        }
+    };
+    if acted {
+        // The next turn's wake prompt tells the agent it was cut off mid-work.
+        interrupted.store(true, std::sync::atomic::Ordering::Relaxed);
+    }
+    bus.emit(crate::events::LiveEvent::Note {
+        text: format!("{by} - {what}"),
+    });
+    acted
+}
+
 /// SIGINT this container's harness-spawned claude child, if any
 /// (best-effort). See [`find_claude_child`] for how it's identified —
 /// **not** a name-based `pkill claude`, which would also hit an unrelated