Add restriction to only run for PRs with main as the target.

.github/workflows/ci.yml

@@ -8,16 +8,32 @@ on:
   pull_request:
 
 jobs:
+  pr-target-test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Fail if target is not staging (for PRs only)
+        if: github.event_name == 'pull_request'
+        run: |
+          if [[ "${{ github.event.pull_request.base.ref }}" != "main" || "${{ github.event.pull_request.head.ref }}" == "staging" ]]; then
+            echo "Target branch is acceptable."
+          else
+            echo "Only PRs from staging can target main."
+            exit 1
+          fi
+
   tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+
+      - name: Checkout code
+        uses: actions/checkout@v2
+
       - name: Set up Python 3.11
         uses: actions/setup-python@v2
         with:
           python-version: 3.11
 
-      - name: run pre-commit
+      - name: Run pre-commit
         run: |
           python -m pip install pre-commit
           pre-commit run --all-files

.github/workflows/preview.yml

@@ -2,6 +2,8 @@ name: Deploy PR previews
 
 on:
   pull_request:
+    branches:
+      - main
     types:
       - opened
       - reopened