swarm-ui: a running-subagents menu in the full-screen terminal header

An N running subagents badge opens a list of the agent's subagents, each a
link to its terminal, so you can switch without going back to the agents page.
A subagent counts as running when it published a row in the last 15 minutes;
the quiet ones are listed below it, dimmed. The list under the preview and the
menu now share one fetch hook and one definition of running.

frontend/packages/swarm-ui/test/subagent-list.test.mjs

@@ -0,0 +1,61 @@
+// What counts as a running subagent, and the menu badge's wording. Run by
+// `npm test`, which Node runs with its built-in test runner and type
+// stripping, so the `.ts` module is imported as it is.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  STALE_S,
+  byRecency,
+  isRunning,
+  runningLabel,
+  splitRunning,
+} from "../src/agent/terminal/subagent-list.ts";
+
+const NOW = 1_000_000;
+const sub = (name, ago) => ({
+  name,
+  last_row_unix: ago === null ? null : NOW - ago,
+});
+
+test("a subagent with a recent row is running, a quiet one is not", () => {
+  assert.equal(isRunning(sub("a", 5), NOW), true);
+  assert.equal(isRunning(sub("a", STALE_S), NOW), true);
+  assert.equal(isRunning(sub("a", STALE_S + 1), NOW), false);
+});
+
+test("a subagent whose time could not be read is not counted as running", () => {
+  assert.equal(isRunning(sub("a", null), NOW), false);
+});
+
+test("newest first, unreadable last, ties by name", () => {
+  const list = [sub("c", 50), sub("z", null), sub("b", 10), sub("a", 10)];
+  assert.deepEqual(
+    [...list].sort(byRecency).map((s) => s.name),
+    ["a", "b", "c", "z"],
+  );
+});
+
+test("the list splits into running and quiet, each newest first", () => {
+  const { running, quiet } = splitRunning(
+    [
+      sub("old", STALE_S + 100),
+      sub("new", 1),
+      sub("mid", 60),
+      sub("none", null),
+    ],
+    NOW,
+  );
+  assert.deepEqual(
+    running.map((s) => s.name),
+    ["new", "mid"],
+  );
+  assert.deepEqual(
+    quiet.map((s) => s.name),
+    ["old", "none"],
+  );
+});
+
+test("the badge counts in words", () => {
+  assert.equal(runningLabel(1), "1 running subagent");
+  assert.equal(runningLabel(4), "4 running subagents");
+});