Load testing foundations: YAML contract, engine levers, CLI load mode, run-flows Action (Phase 0+1) - #45

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-go@v5, actions/setup-node@v4, actions/upload-artifact@v4, nix-community/cache-nix-action@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/