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

tests/service.spec.ts

@@ -77,6 +77,57 @@ describe('BetterSidebar service', () => {
   })
 })
 
+describe('enable switches (declarative settings)', () => {
+  /** A fresh store + service with one tab and one viewer registered. */
+  const setup = () => {
+    const store = createSidebarStore()
+    const service = createBetterSidebarService(store)
+    service.registerTab({ id: 'explorer', title: 'Explorer', component: () => null })
+    service.registerFileViewer({ id: 'image', exts: ['png'], fetchStrategy: 'mediaUrl', component: () => null })
+    return { store, service }
+  }
+
+  it('an absent map key means enabled (default state)', () => {
+    const { service } = setup()
+    expect(service.isTabEnabled('explorer')).toBe(true)
+    expect(service.isViewerEnabled('image')).toBe(true)
+    // Unknown ids are enabled too (nothing registered → the check is open).
+    expect(service.isTabEnabled('whatever')).toBe(true)
+  })
+
+  it('only an explicit false disables a tab type', () => {
+    const { store, service } = setup()
+    store.setPrefs({ ...store.getPrefs(), tabsEnabled: { explorer: false } })
+    expect(service.isTabEnabled('explorer')).toBe(false)
+    store.setPrefs({ ...store.getPrefs(), tabsEnabled: { explorer: true } })
+    expect(service.isTabEnabled('explorer')).toBe(true)
+  })
+
+  it('openTab refuses a disabled tab type (no tab lands, no createTab)', () => {
+    const { store, service } = setup()
+    store.setPrefs({ ...store.getPrefs(), tabsEnabled: { explorer: false } })
+    store.setSession('s1')
+    service.openTab({ type: 'explorer', title: 'Explorer' })
+    const tabs = allLeaves(store.getSnapshot().state!.splits).flatMap(l => l.tabs)
+    expect(tabs).toHaveLength(0)
+  })
+
+  it('matchFileViewer skips a disabled viewer (files fall through)', () => {
+    const { store, service } = setup()
+    service.registerFileViewer({ id: 'code', exts: [], priority: -100, fetchStrategy: 'fsRead', component: () => null })
+    // image claims .png; disabling it lets the catch-all take over.
+    expect(service.matchFileViewer('photo.png')?.id).toBe('image')
+    store.setPrefs({ ...store.getPrefs(), viewersEnabled: { image: false } })
+    expect(service.matchFileViewer('photo.png')?.id).toBe('code')
+    // Disabling the catch-all too → no viewer at all.
+    store.setPrefs({ ...store.getPrefs(), viewersEnabled: { image: false, code: false } })
+    expect(service.matchFileViewer('photo.png')).toBeUndefined()
+    // Re-enabling restores the image viewer.
+    store.setPrefs({ ...store.getPrefs(), viewersEnabled: {} })
+    expect(service.matchFileViewer('photo.png')?.id).toBe('image')
+  })
+})
+
 describe('matchFileViewer', () => {
   it('matches by extension', () => {
     const store = createSidebarStore()