Update GitHub Actions for Node 24

[fazpu]

Summary

• update checkout, setup-uv, upload-artifact, and download-artifact to current stable majors that support the Node 24 runtime
• keeps the workflow-level Node 24 opt-in from Opt GitHub Actions into Node 24 #69

Validation

• git diff --check
• uv run python -c "import yaml, pathlib; [yaml.safe_load(p.read_text()) for p in pathlib.Path('.github/workflows').glob('*.yml')]; print('workflow yaml ok')"