ci(sync-auto): update sync workflow to checkout default branch

- add explicit ref parameter to checkout action for clarity and reliability
- ensure workflow always checks out the repository's default branch

.github/workflows/sync-auto.yml

@@ -45,6 +45,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
+          ref: ${{ github.event.repository.default_branch }}
 
       - name: 检测默认分支
         run: |
@@ -114,6 +115,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
+          ref: ${{ github.event.repository.default_branch }}
 
       - name: 检测默认分支
         run: |