feat: 任务管理页二级开关「有新后台任务时自动展开」（autoOpenTasks）

- 新 pref autoOpenTasks（默认开）：当前会话出现新后台任务（任意新任务 id，
  非仅首个）时自动展开侧边栏并打开任务管理页，与 autoOpenSubagent 并列
- 注册为 subagent tab 的声明式二级开关（设置页齿轮弹窗，key 在 PrefsSchema）
- Sidebar 新增独立基线 effect（镜像子代理自动展开；刷新不触发、开关关闭/类型
  禁用不触发）；detectNewTask 纯函数（settling/同 id/他会话不触发）
- 测试：detectNewTask 用例、prefs 解析断言、builtins 开关清单、schema 默认值
  断言；全量 371 通过

src/client/Sidebar.tsx

@@ -44,6 +44,7 @@ import type { TabDragPayload } from './TabBar.tsx'
 import { relativeTo } from './paths.ts'
 import { OrphanedTab } from './OrphanedTab.tsx'
 import { detectNewDirectSubagent } from './subagent-detect.ts'
+import { detectNewTask } from './subagent-tasks.ts'
 import { t } from './locales.ts'
 import { api, type SessionScope } from './api.ts'
 import css from './sidebar.module.css'
@@ -241,6 +242,28 @@ export function Sidebar(props: { ctx: Context; store: SidebarStore }) {
     ctx.betterSidebar?.openTab({ type: 'subagent', title: t('subagent') })
   }, [sessionList, sessionId, store, ctx])
 
+  /**
+   * Task auto-activation: the moment a NEW background task appears for the
+   * current conversation (a task id the previous snapshot lacked), the
+   * auto-open pref is on, and the Tasks tab type is enabled, open the panel
+   * (if collapsed) and focus the Tasks page. Unlike the subagent trigger
+   * (0 → N only), ANY new task id triggers: the agent may start several
+   * tasks in one session, and each should surface. A fresh page load never
+   * triggers — its baseline starts at the current snapshot.
+   */
+  const taskBaselineRef = useRef<SidebarSessionList | undefined>(undefined)
+  useEffect(() => {
+    const prev = taskBaselineRef.current
+    taskBaselineRef.current = sessionList
+    if (sessionId === undefined || prev === undefined) return
+    if (!detectNewTask(prev, sessionList, sessionId)) return
+    if (!store.getPrefs().autoOpenTasks) return
+    if (ctx.betterSidebar?.isTabEnabled('subagent') === false) return
+    store.reduce(s => s.panelOpen ? s : togglePanel(s))
+    store.reduce(s => ({ ...s, activePane: firstLeaf(s.splits).id }))
+    ctx.betterSidebar?.openTab({ type: 'subagent', title: t('subagent') })
+  }, [sessionList, sessionId, store, ctx])
+
   /**
    * Topology jump-back: clicking a subagent node on the Subagent page calls
    * the official `openSubagent`, which switches the sidebar to that child