fix(git): git.show 路由改用仓库相对路径，修复折叠展开空白

- src/index.ts: git.show 的 path 不再经 resolveGitPath 转绝对路径——rev:path 语法只认仓库相对路径（diff 输出剥 a// b/ 前缀的形态），绝对路径恒 fatal 返 null，折叠展开渲染空行；透传后暴露面与 git.diff/git.log 一致（仅本仓库版本树）
- src/client/changes/DiffPane.tsx / src/client/DiffTab.tsx: 两侧内容皆空时抛错走「上下文未加载」降级，不再静默空白展开

背景：真机 link 测试发现展开后 marker 消失但零行渲染——git.show 路由此前无真实调用方，resolveGitPath 的工作区语义与 rev:path 树寻址不匹配的缝隙首次暴露。
设计 docs/plans/2026-09-07-git-fold-expand-design.md 实施偏差第 8 条同提交。

src/client/DiffTab.tsx

@@ -100,6 +100,14 @@ export function DiffTab(props: { sessionId: string; cwd: string | undefined; dif
   useEffect(() => { foldContents.current = new Map() }, [diff, tick])
   const foldLoader = useMemo(() => {
     const sidesOf = (file: DiffFile): Promise<{ old: string; new: string }> => {
+      // Both sides empty cannot cover a non-empty fold — treat it as a failed
+      // fetch so the fold degrades to the unavailable marker instead of
+      // silently expanding to nothing (the symptom of a bad rev or path
+      // reading null on both sides).
+      const ofSides = (oldContent: string | null, newContent: string | null): { old: string; new: string } => {
+        if ((oldContent ?? '') === '' && (newContent ?? '') === '') throw new Error('no content on either side')
+        return { old: oldContent ?? '', new: newContent ?? '' }
+      }
       const fetchSides = async (): Promise<{ old: string; new: string }> => {
         const scope: SessionScope = { sessionId, cwd, ...(diff.repoRoot !== undefined ? { repoRoot: diff.repoRoot } : {}) }
         if (diff.kind === 'commit') {
@@ -113,7 +121,7 @@ export function DiffTab(props: { sessionId: string; cwd: string | undefined; dif
               ? Promise.resolve({ content: null })
               : api.gitShow(scope, diff.hashFull, displayPath(file.newPath), diff.worktree),
           ])
-          return { old: oldSide.content ?? '', new: newSide.content ?? '' }
+          return ofSides(oldSide.content, newSide.content)
         }
         // Worktree change: staged is HEAD vs index, unstaged is index vs
         // worktree (the worktree side reads the live file).
@@ -127,7 +135,7 @@ export function DiffTab(props: { sessionId: string; cwd: string | undefined; dif
               ? Promise.resolve({ content: null })
               : api.gitShow(scope, ':0', displayPath(file.newPath), diff.worktree),
           ])
-          return { old: oldSide.content ?? '', new: newSide.content ?? '' }
+          return ofSides(oldSide.content, newSide.content)
         }
         const [oldSide, worktree] = await Promise.all([
           file.oldPath === '/dev/null'
@@ -135,7 +143,7 @@ export function DiffTab(props: { sessionId: string; cwd: string | undefined; dif
             : api.gitShow(scope, ':0', displayPath(file.oldPath), diff.worktree),
           api.fsRead(scope, resolveSidebarPath(diff.repoRoot ?? diff.worktree ?? cwd, displayPath(file.newPath))).catch(() => null),
         ])
-        return { old: oldSide.content ?? '', new: worktree !== null && worktree.kind === 'text' ? worktree.content : '' }
+        return ofSides(oldSide.content, worktree !== null && worktree.kind === 'text' ? worktree.content : null)
       }
       const path = displayPath(file.newPath === '/dev/null' ? file.oldPath : file.newPath)
       let promise = foldContents.current.get(path)

src/client/changes/DiffPane.tsx

@@ -236,6 +236,14 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
   const foldLoader = useMemo(() => {
     if (gitRef === null) return undefined
     const sidesOf = (file: DiffFile): Promise<{ old: string; new: string }> => {
+      // Both sides empty cannot cover a non-empty fold — treat it as a failed
+      // fetch so the fold degrades to the unavailable marker instead of
+      // silently expanding to nothing (the symptom of a bad rev or path
+      // reading null on both sides).
+      const ofSides = (oldContent: string | null, newContent: string | null): { old: string; new: string } => {
+        if ((oldContent ?? '') === '' && (newContent ?? '') === '') throw new Error('no content on either side')
+        return { old: oldContent ?? '', new: newContent ?? '' }
+      }
       const fetchSides = async (): Promise<{ old: string; new: string }> => {
         if (gitRef.kind === 'commit') {
           // The patch's -m --first-parent shape: old side from the parent,
@@ -248,7 +256,7 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
               ? Promise.resolve({ content: null })
               : api.gitShow(gitScope, gitRef.hashFull, displayPath(file.newPath), gitRef.worktree),
           ])
-          return { old: oldSide.content ?? '', new: newSide.content ?? '' }
+          return ofSides(oldSide.content, newSide.content)
         }
         // Worktree change: staged is HEAD vs index, unstaged is index vs
         // worktree (the worktree side reads the live file).
@@ -262,7 +270,7 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
               ? Promise.resolve({ content: null })
               : api.gitShow(gitScope, ':0', displayPath(file.newPath), gitRef.worktree),
           ])
-          return { old: oldSide.content ?? '', new: newSide.content ?? '' }
+          return ofSides(oldSide.content, newSide.content)
         }
         const [oldSide, worktree] = await Promise.all([
           file.oldPath === '/dev/null'
@@ -270,7 +278,7 @@ export function DiffPane({ target, scope, height, onHeightCommit, onClose, onExp
             : api.gitShow(gitScope, ':0', displayPath(file.oldPath), gitRef.worktree),
           api.fsRead(gitScope, resolveSidebarPath(gitRef.repoRoot ?? gitRef.worktree ?? scope.cwd, displayPath(file.newPath))).catch(() => null),
         ])
-        return { old: oldSide.content ?? '', new: worktree !== null && worktree.kind === 'text' ? worktree.content : '' }
+        return ofSides(oldSide.content, worktree !== null && worktree.kind === 'text' ? worktree.content : null)
       }
       const path = displayPath(file.newPath === '/dev/null' ? file.oldPath : file.newPath)
       let promise = foldContents.current.get(path)

src/index.ts

@@ -474,7 +474,13 @@ function buildApi(
     'git.show': async (payload) => {
       const { cwd } = await gitCwdOf(payload)
       const repoRoot = selectedRepoOf(payload)
-      const path = await resolveGitPath(cwd, requireString(payload, 'path'), repoRoot)
+      // `git show <rev>:<path>` addresses the path inside the revision TREE:
+      // repository-relative, exactly the unified diff's own path form (after
+      // the a// b/ prefix). The absolute filesystem paths resolveGitPath
+      // produces would break the rev:path syntax and fail every read, so the
+      // path passes through as-is — it can only address blobs of this repo's
+      // own revisions, the same surface git.diff/git.log already expose.
+      const path = requireString(payload, 'path')
       const rev = requireString(payload, 'rev')
       return { content: await git.show(cwd, rev, path, repoRoot) }
     },