ci: add test coverage reporting with Codecov (#5)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: peter-svensson

.github/workflows/ci.yml

@@ -32,4 +32,9 @@ jobs:
         env:
           NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       - run: npm run build
-      - run: npm test
+      - run: npm run test:coverage
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@671740ac38dd9b0130fbe1cec585b89eea48d3de # v5.5.2
+        with:
+          files: ./coverage/coverage-final.json
+          token: ${{ secrets.CODECOV_TOKEN }}