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

tests/unit.spec.ts

@@ -1150,6 +1150,7 @@ describe('side card preferences', () => {
         openByDefault: false,
         defaultWidthPercent: 60,
         autoOpenSubagent: false,
+        autoOpenTasks: true,
         agentTerminalTools: true,
         bottomPanelAutoTerminal: true,
         interceptOpenPath: true,
@@ -1168,6 +1169,7 @@ describe('side card preferences', () => {
         openByDefault: true,
         defaultWidthPercent: 33,
         autoOpenSubagent: true,
+        autoOpenTasks: true,
         agentTerminalTools: false,
         bottomPanelAutoTerminal: true,
         interceptOpenPath: true,
@@ -1186,6 +1188,7 @@ describe('side card preferences', () => {
         openByDefault: false,
         defaultWidthPercent: 40,
         autoOpenSubagent: true,
+        autoOpenTasks: true,
         agentTerminalTools: false,
         bottomPanelAutoTerminal: true,
         interceptOpenPath: true,
@@ -1205,6 +1208,11 @@ describe('side card preferences', () => {
       .toBe(false)
     expect((await loadPrefs(wire({ openByDefault: true, defaultWidthPercent: 40, agentTerminalTools: true }))).agentTerminalTools)
       .toBe(true)
+    // The task auto-open is ON by default; only an explicit false turns it off.
+    expect((await loadPrefs(wire({ openByDefault: true, defaultWidthPercent: 40, autoOpenTasks: 1 }))).autoOpenTasks)
+      .toBe(true)
+    expect((await loadPrefs(wire({ openByDefault: true, defaultWidthPercent: 40, autoOpenTasks: false }))).autoOpenTasks)
+      .toBe(false)
   })
 
   it('defaults interceptOpenPath to true; only an explicit false turns the takeover off', async () => {
@@ -1234,9 +1242,9 @@ describe('side card preferences', () => {
     const store = createSidebarStore()
     // Node environment: no window → the width falls back to PANEL_DEFAULT,
     // while the open flag still follows the preference.
-    store.setPrefs({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
+    store.setPrefs({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, autoOpenTasks: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
     store.setSession('fresh-session')
-    expect(store.getPrefs()).toEqual({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
+    expect(store.getPrefs()).toEqual({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, autoOpenTasks: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
     const snapshot = store.getSnapshot()
     expect(snapshot.sessionId).toBe('fresh-session')
     expect(snapshot.state?.panelOpen).toBe(false)
@@ -1272,7 +1280,7 @@ describe('side card preferences', () => {
 
   it('skips the default explorer tab when the explorer type is disabled', () => {
     const store = createSidebarStore()
-    store.setPrefs({ openByDefault: true, defaultWidthPercent: 30, autoOpenSubagent: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: { explorer: false }, viewersEnabled: {} })
+    store.setPrefs({ openByDefault: true, defaultWidthPercent: 30, autoOpenSubagent: true, autoOpenTasks: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: { explorer: false }, viewersEnabled: {} })
     store.setSession('no-explorer')
     const state = store.getSnapshot().state!
     const tabs = allLeaves(state.splits).flatMap(leaf => leaf.tabs)
@@ -1280,7 +1288,7 @@ describe('side card preferences', () => {
     expect(state.splits.kind).toBe('leaf')
     // Re-enabling seeds the explorer tab again.
     const openStore = createSidebarStore()
-    openStore.setPrefs({ openByDefault: true, defaultWidthPercent: 30, autoOpenSubagent: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
+    openStore.setPrefs({ openByDefault: true, defaultWidthPercent: 30, autoOpenSubagent: true, autoOpenTasks: true, agentTerminalTools: false, bottomPanelAutoTerminal: true, interceptOpenPath: true, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, tabsEnabled: {}, viewersEnabled: {} })
     openStore.setSession('with-explorer')
     const openTabs = allLeaves(openStore.getSnapshot().state!.splits).flatMap(leaf => leaf.tabs)
     expect(openTabs.map(tab => tab.type)).toEqual(['explorer'])