ci: fix LMSYS blog sync to open a PR via gh and only run on main (#27179)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/sync-lmsys-sglang-blogs.yml

@@ -7,9 +7,11 @@ on:
 
 permissions:
   contents: write
+  pull-requests: write
 
 jobs:
   sync:
+    if: github.repository == 'sgl-project/sglang' && github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository
@@ -25,16 +27,34 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: python docs_new/scripts/update_lmsys_sglang_blogs.py
 
-      - name: Commit and push changes
+      - name: Open or update pull request
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          set -euo pipefail
+
           if git diff --quiet -- docs_new/index.mdx; then
-            echo "No blog card changes to commit."
+            echo "No blog card changes; nothing to do."
             exit 0
           fi
 
+          BRANCH="auto/lmsys-blog-cards"
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
+
+          # Recreate the bot branch as "current main + card diff" and overwrite
+          # any previous state, so the PR always shows a clean single-commit diff.
+          git checkout -B "$BRANCH"
           git add docs_new/index.mdx
-          git diff --cached --quiet && exit 0
           git commit -m "docs: sync LMSYS SGLang blog cards"
-          git push
+          git push --force origin "$BRANCH"
+
+          # Open a PR only if one is not already open for this branch; otherwise
+          # the force-push above has already updated the existing PR in place.
+          if [ -z "$(gh pr list --head "$BRANCH" --base main --state open --json number --jq '.[].number')" ]; then
+            gh pr create \
+              --base main \
+              --head "$BRANCH" \
+              --title "docs: sync LMSYS SGLang blog cards" \
+              --body 'Automated sync of the LMSYS SGLang blog cards in `docs_new/index.mdx`, opened by the `Sync LMSYS SGLang blogs` workflow.'
+          fi