Creates .gitea/workflows/build-nixos.yml that triggers on push/PR to
master when Nix files, flake.lock, secrets, hosts, or modules change.
Includes:
- Setup step to enable Nix experimental features and install nh
- Build step for lazyworkhorse (x86_64-linux) via nh os build
- Build step for cyt-pi (aarch64-linux) via nh os build
- Placeholder integration test step
Runner label: nixos-builder (Docker image nixos/nix)