operator control: /cancel slash command + cancel button

new POST /api/cancel on the per-agent web UI: shells out
pkill -INT claude (procps added to harness-base.nix). emits a Note
on the bus so the operator sees the cancel landed; state goes back
to idle when run_claude wakes and emits TurnEnd as usual.

frontend:
- /cancel slash command in the terminal input
- ■ cancel turn button in the state row, visible only while
  state === 'thinking' (driven from the same SSE-based state
  machine). disabled briefly during the POST.

claude gets SIGINT (not TERM) so it flushes anything in-flight and
emits a final result row before exiting.

hive-ag3nt/src/web_ui.rs

@@ -79,6 +79,7 @@ pub async fn serve(
         .route("/login/start", post(post_login_start))
         .route("/login/code", post(post_login_code))
         .route("/login/cancel", post(post_login_cancel))
+        .route("/api/cancel", post(post_cancel_turn))
         .with_state(state);
     let addr = SocketAddr::from(([0, 0, 0, 0], port));
     let listener = tokio::net::TcpListener::bind(addr)
@@ -273,6 +274,33 @@ async fn post_login_cancel(State(state): State<AppState>) -> Response {
     Redirect::to("/").into_response()
 }
 
+/// Cancel the in-flight claude turn. Coarse-grained: shells out
+/// `pkill -INT claude` since there's at most one claude per container.
+/// SIGINT (not SIGTERM) so claude flushes anything in-flight and emits a
+/// final result row. Emits a Note so the operator sees the cancel
+/// landed; the actual state transition back to `idle` happens when
+/// `run_claude` wakes up and the harness emits `TurnEnd`.
+async fn post_cancel_turn(State(state): State<AppState>) -> Response {
+    let out = tokio::process::Command::new("pkill")
+        .args(["-INT", "claude"])
+        .output()
+        .await;
+    let note = match out {
+        Ok(o) if o.status.success() => "operator: /cancel — sent SIGINT to claude".to_owned(),
+        Ok(o) if o.status.code() == Some(1) => {
+            "operator: /cancel — no claude process to interrupt".to_owned()
+        }
+        Ok(o) => format!(
+            "operator: /cancel — pkill exited {} stderr={}",
+            o.status,
+            String::from_utf8_lossy(&o.stderr).trim()
+        ),
+        Err(e) => format!("operator: /cancel — pkill failed: {e}"),
+    };
+    state.bus.emit(crate::events::LiveEvent::Note(note));
+    Redirect::to("/").into_response()
+}
+
 fn error_response(message: &str) -> Response {
     // Plain text — JS app surfaces in `alert()`, HTML wrapping would just
     // be noise.