ci: upload codecov only on main pushes

.github/workflows/ci.yml

@@ -67,6 +67,7 @@ jobs:
         run: bun run test:coverage
 
       - name: Upload coverage to Codecov
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: codecov/codecov-action@v5
         with:
           files: ./coverage/lcov.info