fix(ci): auto-approve使用PR head SHA而非merge SHA检查CI状态

pull_request事件中GITHUB_SHA是merge commit的虚拟SHA，CI实际运行的是head SHA。
用merge SHA查status永远查不到，导致auto-approve一直看到pending，20分钟超时后退出，
永远打不了审批。改用github.event.pull_request.head.sha查询。

.gitea/workflows/auto-approve.yml

@@ -20,6 +20,7 @@ jobs:
           GITHUB_TOKEN: ${{ github.token }}
           REVIEW_TOKEN: ${{ secrets.REVIEW_GITEA_TOKEN }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
+          PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         run: |
           set -eu
 
@@ -79,7 +80,7 @@ jobs:
 
             # 调用辅助脚本检查每个context状态
             for ctx in "${CONTEXTS[@]}"; do
-              STATE=$(python3 scripts/check_ci_status.py "$GITHUB_TOKEN" "$GITHUB_REPOSITORY" "$GITHUB_SHA" "$ctx")
+              STATE=$(python3 scripts/check_ci_status.py "$GITHUB_TOKEN" "$GITHUB_REPOSITORY" "$PR_HEAD_SHA" "$ctx")
               echo "  $ctx: $STATE"
 
               if [ "$STATE" != "success" ]; then