feat(settings): 侧边栏设置声明式化 — 注册表驱动的开关清单 + 内置支持插头化

- SidebarPrefs 新增 tabsEnabled / viewersEnabled 开放 map（缺省 = 启用），
  PrefsSchema 用 z.dict 校验，旧文档零迁移
- TabDescriptor/FileViewerDescriptor 新增声明式设置面：viewer 补 title/icon，
  两者补 settings.toggles（子代理声明 autoOpenSubagent、终端声明 agentTerminalTools）
- 服务新增 isTabEnabled/isViewerEnabled；openTab 拒绝禁用类型、matchFileViewer
  跳过禁用 viewer
- SideCardSection 重写为注册表驱动清单：侧边栏内容（图标+类型id+开关+嵌套开关）
  + 文件预览（图标+标题+扩展名+开关），开关持久化 + 乐观提交回滚
- 消费点 gating：+ 菜单过滤、子代理自动展开、agent 终端 reconcile、
  产出文件拦截（editor 关闭时回落默认行）、新会话 explorer 预置
- 内置 viewer 补 6 个手绘 outline 图标；store 快照携带 prefs 使开关变更即时生效
- 测试：service gating / builtins 声明 / parsePrefs map 校验 / schema 断言 /
  新增 SideCardSection renderToString 规格
- 文档：README 设置小节、AGENTS.md 描述符与服务方法、新增设计文档；版本 0.5.0

src/client/builtins/tabs.tsx

@@ -80,6 +80,15 @@ export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
       icon: (size: number) => <IconThinkOutline16 size={size} />,
       order: 30,
       single: true,
+      // Declarative settings: the auto-open switch renders under this row in
+      // the Side card settings page (the Subagent page's own related setting).
+      settings: {
+        toggles: [{
+          key: 'autoOpenSubagent',
+          title: () => t('settingsSubagentTitle'),
+          desc: () => t('settingsSubagentDesc'),
+        }],
+      },
       component: ({ ctx, scope, visible, onSubagentJump }) => (
         <SubagentView
           sessionId={scope.sessionId}
@@ -95,6 +104,16 @@ export function builtinTabs(ctx: Context): readonly TabDescriptor[] {
       icon: (size: number) => <IconTerminalOutline16 size={size} />,
       order: 40,
       available: (_ctx, _scope, state) => uiTerminalCount(state) < TERMINAL_LIMIT,
+      // Declarative settings: the model-facing terminal tools switch renders
+      // under this row in the Side card settings page (the Terminal page's
+      // own related setting; the host gates the toolset on it independently).
+      settings: {
+        toggles: [{
+          key: 'agentTerminalTools',
+          title: () => t('settingsToolsTitle'),
+          desc: () => t('settingsToolsDesc'),
+        }],
+      },
       createTab: (state) => {
         const count = uiTerminalCount(state)
         if (count >= TERMINAL_LIMIT) return null