feat: add dotfiles submodule and home-manager config

- Add dotfiles repo as submodule in assets/dotfiles/
- Rewrite home.nix with direct file references instead of stow service
- Remove old custom dotfiles.nix service (replaced by home-manager)
- Clean up services/default.nix import

modules/nixos/services/default.nix

@@ -1,6 +1,5 @@
 {
   imports = [
-    ./dotfiles.nix
     ./systemd
   ];
 }