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.

hive-agent/src/web_ui/mod.rs

@@ -117,6 +117,7 @@ pub async fn serve(
         .route("/api/logout", post(auth::post_logout))
         .route("/api/loose-ends", get(stats::api_loose_ends))
         .route("/api/bash-tasks", get(stats::api_bash_tasks))
+        .route("/api/todos", get(stats::api_todos))
         .route("/api/stats", get(stats::api_stats))
         .route("/screen/ws", get(screen::screen_ws))
         .route("/icon", get(screen::serve_icon));