Integrate Home Manager as a NixOS module (flakes-rg27)

Create modules/nixos/settings/home-manager.nix to import home-manager as a NixOS module so nixos-rebuild switch handles both system and home config in one command.

Summary of Changes

Created modules/nixos/settings/home-manager.nix which:

• Imports home-manager.nixosModules.home-manager
• Sets useGlobalPkgs = true and useUserPackages = true
• Passes extraSpecialArgs with the flake argument so home modules get their expected {flake, ...} argument
• Wires home-manager.users.yjpark to import configurations/home/yjpark.nix plus any host-specific mixin from mixins/home/hosts/<hostname>.nix

The module is auto-included in all NixOS hosts via the autowired modules/nixos/settings/ directory. Verified with nix flake check and just build-host — home-manager generation is now built as part of the NixOS system build.