Replace Codecov with Qlty for code coverage reporting

Swap codecov/codecov-action for qltysh/qlty-action/coverage to publish
coverage data to Qlty Cloud. Uses token-based auth (public repo) and
uploads lcov.info from the Node 16 matrix run.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/main.yml

@@ -20,7 +20,8 @@ jobs:
           node-version: ${{ matrix.node-version }}
       - run: npm install
       - run: npm test
-      - uses: codecov/codecov-action@v2
+      - uses: qltysh/qlty-action/coverage@v2
         if: matrix.node-version == 16
         with:
-          fail_ci_if_error: true
+          token: ${{ secrets.QLTY_COVERAGE_TOKEN }}
+          files: coverage/lcov.info