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)