feat(api): 任务改动 diff 端点——GET /api/tasks/:id/diff 按文件切分

结果评审需要看每个文件的具体改动。新增端点跑 git diff defaultBranch...branch，
按 diff --git 切分成 [{path, diff}]，供前端点击文件展开查看。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/server.ts

@@ -9,7 +9,7 @@ import { syncProject, hasTodoJson } from '../sync/todo-sync.js';
 import { resolvedExecutorModels } from '../executor/models.js';
 import { classifyComplexity, type ClassifierFn } from '../executor/classify.js';
 import { acceptAndMerge } from '../executor/exec-merge.js';
-import { createWorktree } from '../executor/worktree.js';
+import { createWorktree, git } from '../executor/worktree.js';
 import { resolveLogo, LOGO_MIME } from './logo.js';
 import { readTranscript, TranscriptError } from '../executor/transcript.js';
 import { createReadStream, createWriteStream, mkdirSync, readFileSync, statSync } from 'node:fs';
@@ -342,6 +342,24 @@ export function buildServer(opts: ApiOptions): FastifyInstance {
     return store.listRuns(id);
   });
 
+  // 任务改动 diff（结果评审用）：git diff defaultBranch...branch，按文件切分返回，供点击文件看具体改动
+  app.get('/api/tasks/:id/diff', async (req) => {
+    const { id } = req.params as { id: string };
+    const task = store.getTask(id);
+    if (!task) throw new StoreError(`任务不存在: ${id}`);
+    const project = store.getProject(task.projectId);
+    const branch = task.result?.branch;
+    if (!project || !branch) return { files: [] };
+    let raw = '';
+    try { raw = await git(project.repoPath, ['diff', `${project.defaultBranch}...${branch}`]); } catch { return { files: [] }; }
+    const files = raw.split(/^diff --git /m).filter(Boolean).map((p) => {
+      const m = p.match(/^a\/(\S+)\s+b\/(\S+)/);
+      const path = m ? m[2] : (p.split('\n')[0] || '').trim();
+      return { path, diff: 'diff --git ' + p };
+    });
+    return { files };
+  });
+
   // 实时流（SSE）：跟随任务「最近一条 run」的 transcript 文件，把新写入的 agent 消息逐行推送。
   // 活跃 run（status=started）= 真实时；已结束 run = 一次性回放后 end。客户端 EventSource 消费。
   app.get('/api/tasks/:id/stream', (req, reply) => {