ci: add GitHub Actions workflow (lint, compile, tests)

.github/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+          cache: pip
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          pip install ruff pytest
+
+      - name: Lint
+        run: ruff check .
+
+      - name: Compile check
+        run: python -m compileall -q .
+
+      - name: Tests
+        run: |
+          if [ -d tests ]; then
+            pytest -q
+          else
+            echo "No tests/ directory yet. See the follow-up issue."
+          fi