Fix Mermaid render: checkout branch ref to avoid detached HEAD on push

Author: dellams

.github/workflows/render-mermaid.yml

@@ -15,6 +15,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
+          ref: ${{ github.head_ref || github.ref_name }}
 
       - name: Setup Node
         uses: actions/setup-node@v4