feat: workspace fence toggle (workspaceFence pref + one-click off in error surfaces)

- New persisted built-in pref workspaceFence (default true) under the
  files card's secondary settings modal; host reads it per request via
  the settings seam (fenceEnabledOf — absent/malformed stays ON).
- ensureWorkspacePath/ensureWorkspaceWritePath gain a fence param:
  disarmed skips the containment assert but keeps realpath resolution;
  wired through fs.tree/fs.read/fs.write, the media + html routes, and
  the upload path (writeWorkspaceUpload fence input).
- Editor load failures and file-tree level errors show the localized
  reason plus a one-click global off (FenceErrorNotice) that flips the
  pref, adopts it into the store, and auto-retries the failed operation
  (reloadSeq / retryDir) — the raw wire text is never shown.
- GitView's two client-side pre-checks (hidden open action + no-op
  defense) respect the pref; store threaded through TreePanel/FileTree.
- i18n: 4 new keys across all 21 dictionaries; tests: fence-off route
  coverage, prefs parsing, schema default, editor toggles list;
  docs: guide §8/§8.1 + plans/2026-08-29 design record.

src/client/locales.ts

@@ -234,6 +234,10 @@ export const zh = {
   settingsJobsDesc: '当前会话出现新的后台任务时，自动展开侧边栏并打开后台任务页（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）',
+  settingsFenceTitle: '工作区路径检测',
+  settingsFenceDesc: '开启后，侧栏的文件功能仅能访问会话工作区内的路径（默认）；关闭后可访问主机上任意文件——关闭期间页面内脚本也将获得同等访问能力',
+  fenceErrorReason: '此路径在会话工作区之外，已被工作区检测拦截',
+  fenceDisableAction: '关闭工作区检测',
   settingsBottomTerminalTitle: '底部面板首次展开自动开终端',
   settingsBottomTerminalDesc: '每次会话中第一次展开底部面板时，尝试在底部面板自动打开一个新终端标签（终端数量上限仍会限制；默认开启）',
   settingsFontFamilyTitle: '终端字体',
@@ -590,6 +594,10 @@ export const en: Record<keyof typeof zh, string> = {
   settingsJobsDesc: 'Expand the side card and open the Jobs page whenever a new background job appears for the current conversation (every new job 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)',
+  settingsFenceTitle: 'Workspace path fence',
+  settingsFenceDesc: 'On, the sidebar\'s file features only reach paths inside the session workspace (default); off, any file on the host is reachable — page scripts gain the same reach while it is off',
+  fenceErrorReason: 'This path is outside the session workspace and was blocked by the workspace fence',
+  fenceDisableAction: 'Turn off the workspace fence',
   settingsBottomTerminalTitle: 'Auto-open a terminal on the bottom panel\'s first expansion',
   settingsBottomTerminalDesc: 'When the bottom panel is expanded for the first time in a session, try to open a fresh terminal tab there (the terminal quota still applies; on by default)',
   settingsFontFamilyTitle: 'Terminal font family',