feat(frontend): pulse the stop dot while the harness is working

Give live feedback that something is happening the moment a session starts
processing: the red stop dot in the composer beats with a soft heartbeat + halo
while the harness is actively working (running — not waiting at a gate or
finalized), and settles the instant the model hands control back to the reviewer
or the session ends. Ring/scale only (no layout properties animated); honors
prefers-reduced-motion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -508,6 +508,7 @@ export function AppShell() {
                   onStop={() => setStopConfirm(true)}
                   inputRef={composerRef}
                   awaitingInput={awaitingQuestion || pendingWidget?.widget === "freetext"}
+                  working={running}
                 />
               </div>
               <ComposerFooter />

frontend/src/shell/SteerInput.test.tsx

@@ -82,3 +82,12 @@ test("marks the composer as awaiting input only when requested", () => {
   expect(input).toHaveAttribute("data-awaiting-input", "true");
   expect(input).toHaveClass("thot-awaiting-input");
 });
+
+test("pulses the stop dot only while the harness is working", () => {
+  const { rerender } = render(<SteerInput sessionId="s1" working />);
+  const dot = () =>
+    screen.getByRole("button", { name: /stop and save session/i }).querySelector("span");
+  expect(dot()).toHaveClass("thot-pulse");
+  rerender(<SteerInput sessionId="s1" working={false} />);
+  expect(dot()).not.toHaveClass("thot-pulse");
+});

frontend/src/shell/SteerInput.tsx

@@ -26,6 +26,8 @@ interface Props {
   inputRef?: React.Ref<HTMLTextAreaElement>;
   /** The workflow is explicitly waiting for text in this composer. */
   awaitingInput?: boolean;
+  /** The harness is actively working: pulse the stop dot to signal live activity. */
+  working?: boolean;
 }
 
 export function SteerInput({
@@ -36,6 +38,7 @@ export function SteerInput({
   onStop,
   inputRef,
   awaitingInput = false,
+  working = false,
 }: Props) {
   const [text, setText] = useState("");
   const [busy, setBusy] = useState(false);
@@ -115,7 +118,10 @@ export function SteerInput({
           aria-label="Stop and save session"
           className="grid size-7 shrink-0 place-items-center rounded-md border border-border text-primary transition-colors hover:bg-muted"
         >
-          <span className="block size-2.5 rounded-full bg-primary" />
+          <span
+            className={["block size-2.5 rounded-full bg-primary", working && "thot-pulse"]
+              .filter(Boolean).join(" ")}
+          />
         </button>
       )}
       <textarea