chore: import upstream snapshot with attribution

.github/workflows/enforce-pullrequest-rules.yaml

@@ -0,0 +1,29 @@
+# This workflow enforces on every pull request that the PR is not based against master,
+# taken from https://github.com/oppia/oppia-android/blob/develop/.github/workflows/static_checks.yml
+
+name: "Enforce Pull-Request Rules"
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+  push:
+    branches-ignore:
+      - develop
+      - master
+
+jobs:
+  check:
+    runs-on: ubuntu-slim
+    if: github.event_name == 'pull_request'
+    timeout-minutes: 10
+    steps:
+      - name: "Branch is not based on develop"
+        if: ${{ github.base_ref != 'develop' && !contains(github.event.pull_request.labels.*.name, 'mastermerge') }}
+        run: |
+          echo "Current base branch: $BASE_BRANCH"
+          echo "Note: PRs should only ever be merged into develop so please rebase your branch on develop and try again."
+          exit 1
+        env:
+          BASE_BRANCH: ${{ github.base_ref }}