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/agent.css

@@ -338,7 +338,7 @@ h2, h3 {
   background: color-mix(in srgb, var(--red) 18%, transparent);
   color: var(--red);
 }
-.header-pill-tasks .header-pill-count {
+.header-pill-todos .header-pill-count {
   background: color-mix(in srgb, var(--green) 18%, transparent);
   color: var(--green);
 }