Replace all Blacksmith runner references with standard GitHub-hosted runners:
- blacksmith-4vcpu-ubuntu-2404 → ubuntu-latest
- blacksmith-4vcpu-ubuntu-2404-arm → ubuntu-24.04-arm
- blacksmith-4vcpu-windows-2025 → windows-latest
This enables GitHub Actions to run in the Kilo-Org fork which doesn't have
access to Blacksmith runners.