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/locales.ts

@@ -121,6 +121,8 @@ const zh = {
   zoomHint: 'Alt + 滚轮',
   settingsSubagentTitle: '检测到子代理时自动展开任务管理页',
   settingsSubagentDesc: '当前会话产生新的子代理时，自动展开侧边栏并打开任务管理页；关闭后需手动打开',
+  settingsTasksTitle: '有新后台任务时自动展开任务管理页',
+  settingsTasksDesc: '当前会话出现新的后台任务时，自动展开侧边栏并打开任务管理页（每个新任务都会触发）；关闭后需手动打开',
   settingsToolsTitle: '为模型注入终端工具',
   settingsToolsDesc: '开启后，模型可通过 terminal_create 等 8 个工具创建并操作侧边栏终端（默认关闭）',
   settingsBottomTerminalTitle: '底部面板首次展开自动开终端',
@@ -194,7 +196,7 @@ const zh = {
   taskViewOutput: '查看输出',
   taskHideOutput: '收起输出',
   taskNoOutput: '暂无输出',
-  taskNotReadYet: '模型尚未读取该任务的输出，读取后会显示在这里',
+  taskNotReadYet: '等待模型读取该任务的输出（模型执行 task_output 后，输出会显示在这里）',
   taskOutputTruncated: '输出过长，已截断显示',
   taskOutputError: '输出读取失败',
   taskKill: '终止',
@@ -318,6 +320,8 @@ const en: Record<keyof typeof zh, string> = {
   zoomHint: 'Alt + wheel',
   settingsSubagentTitle: 'Auto-open the Tasks page when a subagent appears',
   settingsSubagentDesc: 'Expand the side card and open the Tasks page when the current conversation spawns a new subagent; turn off to open it manually',
+  settingsTasksTitle: 'Auto-open the Tasks page on a new background task',
+  settingsTasksDesc: 'Expand the side card and open the Tasks page whenever a new background task appears for the current conversation (every new task triggers); turn off to open it manually',
   settingsToolsTitle: 'Inject terminal tools for the model',
   settingsToolsDesc: 'When enabled, the model can create and drive sidebar terminals through the 8 terminal_* tools (off by default)',
   settingsBottomTerminalTitle: 'Auto-open a terminal on the bottom panel\'s first expansion',
@@ -391,7 +395,7 @@ const en: Record<keyof typeof zh, string> = {
   taskViewOutput: 'View output',
   taskHideOutput: 'Hide output',
   taskNoOutput: 'No output yet',
-  taskNotReadYet: 'The model has not read this task\'s output yet; it will appear here once read',
+  taskNotReadYet: 'Waiting for the model to read this task; its output appears here once the model runs task_output',
   taskOutputTruncated: 'Output truncated',
   taskOutputError: 'Failed to read output',
   taskKill: 'Kill',