ci: add GitHub Actions workflow for automated tests

- Trigger on push to main and on all pull requests
- Run npm test on Node.js 20 (Ubuntu runner)
- Zero-config: leverages existing 'npm test' script

Co-authored-by: Gemini <218195315+gemini-cli@users.noreply.github.com>

Author: jfrelat-lab

.github/workflows/test.yml

@@ -0,0 +1,19 @@
+name: Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Run test suite
+        run: npm test