Add cppcheck and formatter to CI (#19)

* Add cppcheck and formatter to CI

* Checkout submodules

* Install matching clang-format version in CI

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+on: push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          submodules: recursive
+      - uses: actions/cache@v5
+        with:
+          path: |
+            ~/.cache/pip
+            ~/.platformio/.cache
+          key: ${{ runner.os }}-pio
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.14'
+
+      - name: Install PlatformIO Core
+        run: pip install --upgrade platformio
+
+      - name: Install clang-format-21
+        run: |
+          wget https://apt.llvm.org/llvm.sh
+          chmod +x llvm.sh
+          sudo ./llvm.sh 21
+          sudo apt-get update
+          sudo apt-get install -y clang-format-21
+
+      - name: Run cppcheck
+        run: pio check --fail-on-defect medium --fail-on-defect high
+
+      - name: Run clang-format
+        run: PATH="/usr/lib/llvm-21/bin:$PATH" ./bin/clang-format-fix && git diff --exit-code || (echo "Please run 'bin/clang-format-fix' to fix formatting issues" && exit 1)
+
+      - name: Build CrossPoint
+        run: pio run