Add automatic message on submodule modifications

.github/ghprcomment.yml

@@ -91,3 +91,8 @@
 
     Please [merge `upstream/main`](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork#syncing-a-fork-branch-from-the-command-line) with your code.
     For a step-by-step guide to resolve merge conflicts, see <https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/addressing-merge-conflicts/resolving-a-merge-conflict-using-the-command-line>.
+- jobName: 'Submodules not modified'
+  message: >
+    Your pull request modified git submodules.
+
+    Please follow our [FAQ on submodules](https://devdocs.jabref.org/code-howtos/faq.html#submodules) to fix.

.github/workflows/on-pr-opened.yml

@@ -63,6 +63,25 @@ jobs:
           echo "✅ No merge conflicts"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  unmodified_submodules:
+    name: Submodules not modified
+    if: github.actor != 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+          show-progress: 'false'
+      - name: Check for submodule modifications
+        id: check_submodule
+        run: |
+          git fetch origin ${{ github.base_ref }}
+          DIFF=$(git diff --submodule=log origin/${{ github.base_ref }} HEAD)
+          if [ -n "$DIFF" ]; then
+            echo "❌ Submodule modifications detected"
+            exit 1
+          fi
+          echo "✅ No submodule modifications"
   upload-pr-number:
     runs-on: ubuntu-latest
     steps: