Extract the reusable core module list into modules/default.nix (single
source of truth) and expose it as nixosModules.omnixy / .default.
configuration.nix now imports ./modules instead of listing each module,
so this repo's hosts and external consumers share one definition. Theme
stays a settings.theme-driven import inside the aggregate. Packs,
dev-env, sops and home-manager remain injected by mkSystem.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
