Add reusable remote-builder NixOS module and builder system user. Server (lazyworkhorse): dispatches aarch64-linux builds to uConsole uConsole: dispatches x86_64-linux builds to server The builder user uses the same SSH keypair on both hosts for symmetric remote building. Generate the key with: ssh-keygen -t ed25519 -f /etc/ssh/builder_key -N "" Add the public key to lib/keys.nix (replace PLACEHOLDER).