Kieran's opinionated (and probably slightly dumb) nix config

fix: use pre-built zmx binaries to avoid zig2nix dependency master

- Create custom package that downloads zmx binaries from zmx.sh - Avoids zig2nix build issues on GitHub Actions x86_64 runner - Add useGlobalPkgs to home-manager for consistent overlays - Remove nixpkgs.enable from NixOS home configs - Remove vscode config to eliminate nixpkgs.overlays warning 💘 Generated with Crush Assisted-by: Claude Sonnet 4 via Crush <crush@charm.land>