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/prefs-shared.ts

@@ -27,6 +27,23 @@ export interface SidebarPrefs {
    * user explicitly enables it in the side card settings.
    */
   agentTerminalTools: boolean
+  /**
+   * Per-tab enable switches, keyed by tab descriptor id (`'explorer'`,
+   * `'my-plugin:db'`). An ABSENT key means enabled — only an explicit
+   * `false` disables a tab type (hidden from the + menu, `openTab` refuses,
+   * and derived flows like subagent auto-open / agent-terminal tabs stop).
+   * Already-open tabs of a disabled type keep rendering (closing one
+   * prevents reopening), matching the "existing conversations keep their
+   * own layouts" rule.
+   */
+  tabsEnabled: Record<string, boolean>
+  /**
+   * Per-viewer enable switches, keyed by file viewer descriptor id
+   * (`'image'`, `'my-plugin:csv'`). An ABSENT key means enabled; a disabled
+   * viewer is skipped by `matchFileViewer` so files fall through to the
+   * next matching viewer (or the download button when none match).
+   */
+  viewersEnabled: Record<string, boolean>
 }
 
 /** Range contract of {@link SidebarPrefs.defaultWidthPercent}. */
@@ -40,6 +57,8 @@ export const SIDEBAR_PREFS_DEFAULTS: SidebarPrefs = {
   defaultWidthPercent: WIDTH_PERCENT_DEFAULT,
   autoOpenSubagent: true,
   agentTerminalTools: false,
+  tabsEnabled: {},
+  viewersEnabled: {},
 }
 
 /** Clamp one width percent into the contract range (shared by schema and client reads). */