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/builtins/tabs.tsx

@@ -91,9 +91,11 @@ export function builtinTabs(ctx: Context, options: BuiltinTabOptions = {}): read
       dedupeKey: (tab) => tab.path,
       // Declarative settings: the file-open behavior picker (in-place switch
       // vs per-path windows) renders as an iconed select row under the
-      // editor card's gear in the Side card settings page; the "open with"
-      // configuration (SSH host + custom editors) is the custom panel BELOW
-      // those rows — the settings seam renders rows first, custom panel after.
+      // editor card's gear in the Side card settings page, followed by the
+      // workspace fence switch (the host's containment guard over every
+      // sidebar filesystem route); the "open with" configuration (SSH host +
+      // custom editors) is the custom panel BELOW those rows — the settings
+      // seam renders rows first, custom panel after.
       settings: {
         toggles: [{
           key: 'editorExplorer',
@@ -114,6 +116,10 @@ export function builtinTabs(ctx: Context, options: BuiltinTabOptions = {}): read
               desc: () => t('editorExplorerSplitDesc'),
             },
           ],
+        }, {
+          key: 'workspaceFence',
+          title: () => t('settingsFenceTitle'),
+          desc: () => t('settingsFenceDesc'),
         }],
         render: ({ pluginSettings, updatePluginSetting }) => (
           <OpenWithSettings pluginSettings={pluginSettings} updatePluginSetting={updatePluginSetting} />
@@ -141,6 +147,7 @@ export function builtinTabs(ctx: Context, options: BuiltinTabOptions = {}): read
       component: ({ ctx, store, scope, visible, onOpenDiff }) => (
         <GitView
           scope={scope}
+          store={store}
           visible={visible}
           onOpenFile={(path) => { openSidebarFile(ctx, store, scope.sessionId, path) }}
           onOpenDiff={onOpenDiff ?? (() => { /* no-op */ })}