Expert guidance on Nix, NixOS, home-manager, flakes, and Nix packaging. Use when writing or debugging Nix expressions, configuring NixOS or home-manager, authoring derivations, managing flake inputs, encrypting secrets with agenix or sops-nix, or hardening a NixOS host. Also use when committing or reviewing a flake.lock or `nix flake update` result, pinning or overriding a flake input, and when reviewing or deploying a NixOS config change (especially to a remote host) before it is applied. Triggers on Nix, NixOS, nixpkgs, home-manager, flake.nix, flake.lock, nix flake update, nixos-rebuild, nix build, nix develop, devShell, derivation, overlay, agenix, and on Nix errors such as hash mismatches, infinite recursion, or evaluation failures. Advisory rather than generative: it explains, reviews, and debugs. If the repository provides its own skill for authoring host configurations, prefer that for writing config.