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/assets/app.js

@@ -161,10 +161,24 @@
   let termAPI = null;
 
   const SLASH_COMMANDS = [
-    { name: '/help',  desc: 'list slash commands' },
-    { name: '/clear', desc: 'wipe the terminal panel (local-only)' },
+    { name: '/help',   desc: 'list slash commands' },
+    { name: '/clear',  desc: 'wipe the terminal panel (local-only)' },
+    { name: '/cancel', desc: 'SIGINT the in-flight claude turn' },
   ];
 
+  async function postCancelTurn() {
+    try {
+      const resp = await fetch('/api/cancel', { method: 'POST', redirect: 'manual' });
+      const ok = resp.ok || resp.type === 'opaqueredirect'
+        || (resp.status >= 200 && resp.status < 400);
+      if (!ok && termAPI) {
+        termAPI.row('turn-end-fail', '✗ /cancel failed: http ' + resp.status);
+      }
+    } catch (err) {
+      if (termAPI) termAPI.row('turn-end-fail', '✗ /cancel failed: ' + err);
+    }
+  }
+
   function handleSlashCommand(line) {
     if (!termAPI) return false;
     const trimmed = line.trim();
@@ -181,6 +195,9 @@
         termAPI.clear();
         termAPI.row('note', '· terminal cleared (local view only — server history kept)');
         return true;
+      case '/cancel':
+        postCancelTurn();
+        return true;
       default:
         termAPI.row('turn-end-fail', '✗ unknown slash command: ' + cmd + ' — try /help');
         return true;
@@ -282,6 +299,8 @@
     const age = fmtAge(Date.now() - stateSince);
     badge.textContent = def.glyph + ' ' + def.text + ' · ' + age;
     badge.className = 'state-badge state-' + stateName;
+    const cancelBtn = $('cancel-btn');
+    if (cancelBtn) cancelBtn.hidden = stateName !== 'thinking';
   }
   function setState(next) {
     if (next === stateName) return;
@@ -302,6 +321,16 @@
   }
   startStateTicker();
 
+  // Wire the cancel-turn button (visible only while state === thinking).
+  (() => {
+    const btn = $('cancel-btn');
+    if (!btn) return;
+    btn.addEventListener('click', () => {
+      btn.disabled = true;
+      postCancelTurn().finally(() => { btn.disabled = false; });
+    });
+  })();
+
   // Track banner activity by reference-counting in-flight turns. A turn
   // can begin while the previous turn_end is still in the pipeline (rare
   // but happens on tight wake cycles), so we count rather than toggle.