feat(#2632): switch agent page loose-ends pill to todos; remove bash-tasks pill

- Add GET /api/todos backend endpoint (connects to HIVE_AGENT_SOCKET,
  calls ListTodos, returns { todos: [...] } — LooseEnd::Todo items).
- Register route in web_ui/mod.rs.
- index.html: replace loose-ends pill (🪢) + bash-tasks pill (⚙)
  with a single todos pill (📋, id=todos-pill).
- agent.css: add .header-pill-todos count colour (green, same as old tasks).
- app.js:
  - refreshTodos() fetches /api/todos, drives renderTodos/buildTodosList.
  - refreshLooseEnds() becomes background-only (no pill); still drives
    reconcileAskBinds for inline ask-form wiring.
  - Remove refreshBashTasks / buildBashTasksList / renderBashTasks.
  - Cold-load and turn_end both call refreshTodos; 4s interval replaces
    the old bash-tasks interval.

frontend/packages/agent/src/index.html

@@ -58,17 +58,11 @@
         <span class="header-pill-label">inbox</span>
         <span class="header-pill-count" id="inbox-count">0</span>
       </button>
-      <button type="button" id="loose-ends-pill" class="header-pill header-pill-loose" hidden
-              title="open loose-ends flyout">
-        <span class="header-pill-icon" aria-hidden="true">🪢</span>
-        <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 type="button" id="todos-pill" class="header-pill header-pill-todos" hidden
+              title="open todos flyout">
+        <span class="header-pill-icon" aria-hidden="true">📋</span>
+        <span class="header-pill-label">todos</span>
+        <span class="header-pill-count" id="todos-count">0</span>
       </button>
       <button type="button" id="overflow-btn" class="overflow-btn"
               aria-haspopup="menu" aria-expanded="false"