♻️ fix(vscode): 「已查看」改为按文件变更指纹失效，PR 新 push 只摘变动文件的勾

原先按 head sha 存已查看，作者一推新提交 head sha 变了，全部勾一起清空——
哪怕只改了一个文件也把没动的勾全冲掉。改成 GitHub「Viewed」式的内容感知：
按每个文件的变更指纹 (status|additions|deletions) 存，拉文件时与当前指纹比对，
只有真改动过的文件掉勾，没动的保留；比对后顺手剪掉盘上失配/消失的条目。

Gitea 的 ChangedFile 不返回 blob sha、compare 端点也不支持任意 commit sha，
故用上述三元组作指纹（文件一改几乎必变其一），零额外请求。

- prReviewStore: 形状改为 issueNumber → {path: sig}，导出 read/writeConfirmedSigs
- prFiles: handleGetPrFiles 按指纹过滤+剪枝，顺带去掉不再需要的 getPullRequest 调用
- 消息 pr-files/show 去掉无人消费的 headSha/mergeBase；pr-review/set 改发 {path: sig}
- usePrFiles: 删 headSha，setConfirmed 把路径映射成指纹再发

vscode/src/panel/messages.ts

@@ -100,7 +100,7 @@ export type ExtensionToWebview
     | { type: 'issue/remove', issueNumber: number }
     | { type: 'profiles/show', data: ProfilesData }
     | { type: 'managed-sessions/show', data: ManagedSessionsShowData }
-    | { type: 'pr-files/show', issueNumber: number, headSha: string, mergeBase: string, files: PrFile[], confirmed: string[], summaries: Record<string, string>, error?: string }
+    | { type: 'pr-files/show', issueNumber: number, files: PrFile[], confirmed: string[], summaries: Record<string, string>, error?: string }
     | { type: 'pr-file-diff/show', issueNumber: number, path: string, oldContent: string, newContent: string, oldLang?: string, newLang?: string, error?: string }
     | { type: 'pr-file-summary/show', issueNumber: number, path: string, summary?: string, error?: string }
 
@@ -176,4 +176,4 @@ export type WebviewToExtension
     | { type: 'pr-files/get', issueNumber: number }
     | { type: 'pr-file-diff/get', issueNumber: number, path: string, previousPath?: string }
     | { type: 'pr-file-summary/generate', issueNumber: number, path: string, previousPath?: string }
-    | { type: 'pr-review/set', issueNumber: number, headSha: string, confirmed: string[] }
+    | { type: 'pr-review/set', issueNumber: number, confirmed: Record<string, string> }