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/FileTree.tsx

@@ -29,11 +29,13 @@ import {
 } from '@deepseek-ai/dsh-client-ui-primitives'
 import { SiCursor, SiZedindustries } from 'react-icons/si'
 import { VscFile, VscFolder, VscFolderOpened, VscLinkExternal, VscPin, VscPinned } from 'react-icons/vsc'
-import { api, downloadUrl, type FsEntry } from './api.ts'
+import { api, downloadUrl, isOutsideWorkspaceMessage, type FsEntry } from './api.ts'
+import { FenceErrorNotice } from './FenceErrorNotice.tsx'
 import { IconUploadOutline16, IconVscode16 } from './icons.tsx'
 import type { OpenWithTarget } from './open-with.ts'
 import { relativeTo } from './paths.ts'
 import { t } from './locales.ts'
+import type { SidebarStore } from './state.ts'
 import { uploadItemsFromDrop, uploadItemsFromFiles, type UploadItem } from './upload.ts'
 import css from './sidebar.module.css'
 
@@ -105,6 +107,8 @@ const ChatDropIllustration = () => (
 export function FileTree(props: {
   sessionId: string
   cwd: string | undefined
+  /** The sidebar store: the fence-refusal notice writes the `workspaceFence` pref through it. */
+  store: SidebarStore
   expanded: string[]
   /** Files highlighted by a "Show in folder" reveal (absolute paths). */
   revealed: string[]
@@ -136,7 +140,7 @@ export function FileTree(props: {
   /** True while an upload is in flight (drops are ignored). */
   busy: boolean
 }) {
-  const { sessionId, cwd, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, refreshTick, onUploadRequest, busy } = props
+  const { sessionId, cwd, store, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, refreshTick, onUploadRequest, busy } = props
   const [data, setData] = useState<Record<string, LevelData>>({})
   const dataRef = useRef(data)
   /** The row whose path was just copied ("copied" label replaces its button). */
@@ -257,6 +261,13 @@ export function FileTree(props: {
     })
   }, [sessionId, cwd, storeLevel])
 
+  /** Drop one level from the cache and reload it (the fence notice's retry). */
+  const retryDir = useCallback((dir: string) => {
+    delete dataRef.current[dir]
+    setData({ ...dataRef.current })
+    loadDir(dir)
+  }, [loadDir])
+
   // The caller's refresh tick wipes the cache (declared BEFORE the load
   // effect so the reload below sees the empty cache).
   const lastTick = useRef(refreshTick)
@@ -436,6 +447,16 @@ export function FileTree(props: {
       return <div className={css.explorerRow} style={{ paddingLeft: depth * 22 + 6 }}>{t('loading')}</div>
     }
     if (level.error !== undefined) {
+      // The fence refusal becomes the friendly notice (reason + one-click
+      // global off + immediate retry of this directory), never the raw
+      // `path "..." is outside workspace` wire text.
+      if (isOutsideWorkspaceMessage(level.error)) {
+        return (
+          <div style={{ paddingLeft: depth * 22 + 6 }}>
+            <FenceErrorNotice store={store} onDisabled={() => { retryDir(dir) }} />
+          </div>
+        )
+      }
       return (
         <div className={clsx(css.explorerRow, css.explorerError)} style={{ paddingLeft: depth * 22 + 6 }}>
           {level.error}