feat: comprehensive NixOS deployment infrastructure
- docs/nix-container-install.md: 474-line guide covering Determinate Systems
installer, vanilla Nix, NixOS base image, architecture notes (x86_64 vs aarch64),
cross-compilation, container considerations, troubleshooting
- scripts/deploy.sh: 286-line deployment script with pre-flight checks, git sync,
build validation (nix build --no-link), 5 actions (switch/boot/test/build/
dry-activate), color-coded logging, env-based configurability
- scripts/deploy-ssh-config: SSH config for all 3 hosts with dual users for
lazyworkhorse, reverse tunnel for cyt-pi, uConsole placeholder, Gitea entry
Full replacements of stub files from previous commit.