Move CI-only check-format script to .github/

.github/workflows/check-format.yml

@@ -0,0 +1,21 @@
+name: Code format checking
+on: pull_request
+
+env:
+  # Force colored output (see https://bixense.com/clicolors/ and https://force-color.org/)
+  TERM: xterm-256color
+  CLICOLOR: 1
+  CLICOLOR_FORCE: 1
+
+jobs:
+  check-format:
+    runs-on: ubuntu-slim
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v7
+      - name: Install clang-format
+        run: |
+          sudo apt install -Uyq clang-format
+      - name: Check format
+        run: |
+          .github/scripts/check-format.sh