agent-ui: running bash-tasks pill + flyout on the agent page

Adds a 'tasks' header pill (hidden at zero, like inbox/loose-ends) that
opens a side-panel flyout listing the agent's in-flight bash tasks from
GET /api/bash-tasks. Each row shows status (running/queued), task id,
elapsed time, and a truncated single-line command preview. Polled on the
same /api/state cycle as loose-ends (tasks complete async between turns, so
the count stays live); clicking the pill opens the flyout. Snapshot-only
v1 — SSE live-push is a possible follow-up.

frontend/packages/agent/src/index.html

@@ -64,6 +64,12 @@
         <span class="header-pill-label">loose ends</span>
         <span class="header-pill-count" id="loose-ends-count">0</span>
       </button>
+      <button type="button" id="bash-tasks-pill" class="header-pill header-pill-tasks" hidden
+              title="open running bash-tasks flyout">
+        <span class="header-pill-icon" aria-hidden="true">⚙</span>
+        <span class="header-pill-label">tasks</span>
+        <span class="header-pill-count" id="bash-tasks-count">0</span>
+      </button>
       <button type="button" id="overflow-btn" class="overflow-btn"
               aria-haspopup="menu" aria-expanded="false"
               title="more actions">⋯</button>