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/agent.css

@@ -126,6 +126,26 @@ pre.diff {
 }
 #state-row {
   margin: 0.4em 0 0.2em;
+  display: flex;
+  align-items: center;
+  gap: 0.6em;
+}
+.btn-cancel-turn {
+  font-family: inherit;
+  font-size: 0.8em;
+  letter-spacing: 0.08em;
+  background: transparent;
+  color: var(--red);
+  border: 1px solid var(--red);
+  border-radius: 999px;
+  padding: 0.2em 0.8em;
+  cursor: pointer;
+  text-shadow: 0 0 4px currentColor;
+  transition: box-shadow 0.15s ease, background 0.15s ease;
+}
+.btn-cancel-turn:hover {
+  background: rgba(243, 139, 168, 0.1);
+  box-shadow: 0 0 10px -2px currentColor;
 }
 .state-badge {
   display: inline-block;