CI: add Codecov token for coverage upload

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

Author: dsarno

.github/workflows/ci.yml

@@ -39,6 +39,7 @@ jobs:
         uses: codecov/codecov-action@v5
         with:
           files: ./coverage.xml
+          token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Lint
         run: ruff check src/ tests/