fix: use token for authentication to support private repos

.gitea/workflows/autograde.yml

@@ -53,10 +53,15 @@ jobs:
           echo "✅ Gradle configured to use Aliyun mirror"
       
       - name: Checkout code
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
         run: |
           git config --global --add safe.directory ${{ github.workspace }}
           git init
-          git remote add origin ${{ github.server_url }}/${{ github.repository }}.git
+          # Use token for authentication (required for private repos)
+          REPO_URL="${{ github.server_url }}/${{ github.repository }}.git"
+          AUTH_URL=$(echo "$REPO_URL" | sed "s|://|://${GITHUB_TOKEN}@|")
+          git remote add origin "$AUTH_URL"
           git fetch --depth=1 origin ${{ github.sha }}
           git checkout ${{ github.sha }}