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/prefs.ts

@@ -21,8 +21,7 @@ export type { SidebarPrefs }
 /** The settings wire face the preferences need (a subset of the plugin api). */
 export type SidebarSettingsClient = Pick<typeof api, 'settingsGet' | 'settingsUpdate'>
 
-/**
- * Validate one raw resolved value into {@link SidebarPrefs}. Used for the
+/** Validate one raw resolved value into {@link SidebarPrefs}. Used for the
  * settings.get payload AND the settings.update response (both carry the
  * layered resolved value); any malformed field falls back to its default.
  * @param value - the raw resolved section from the settings wire.
@@ -44,9 +43,25 @@ export function parsePrefs(value: unknown): SidebarPrefs {
     agentTerminalTools: typeof record.agentTerminalTools === 'boolean'
       ? record.agentTerminalTools
       : SIDEBAR_PREFS_DEFAULTS.agentTerminalTools,
+    tabsEnabled: booleanMapOf(record.tabsEnabled),
+    viewersEnabled: booleanMapOf(record.viewersEnabled),
   }
 }
 
+/**
+ * Validate one enable-switch map (per-tab / per-viewer). Only boolean values
+ * survive; a non-object or a non-boolean entry falls back to the empty map /
+ * drops the entry — an absent key means the feature stays enabled.
+ */
+function booleanMapOf(value: unknown): Record<string, boolean> {
+  if (value === null || typeof value !== 'object' || Array.isArray(value)) return {}
+  const out: Record<string, boolean> = {}
+  for (const [key, item] of Object.entries(value as Record<string, unknown>)) {
+    if (typeof item === 'boolean') out[key] = item
+  }
+  return out
+}
+
 /**
  * Read the resolved side card preferences through the plugin's settings route.
  * @param settings - the settings wire face (the plugin api by default).