Scaffold nikita-nixos host

Add a second nixosConfigurations entry for a friend's machine running
Plasma 6 + SDDM. The hardware-configuration.nix is a placeholder; it
must be replaced with the output of nixos-generate-config on the
actual hardware before installing. Git identity in hosts/nikita-nixos/
home.nix is also a placeholder.

system/display-managers/sddm.nix

@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  services.displayManager.sddm.enable = true;
+}

system/plasma.nix

@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  services.desktopManager.plasma6.enable = true;
+}

system/users/nikita.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+{
+  users.users.nikita = {
+    isNormalUser = true;
+    extraGroups = [ "wheel" "networkmanager" ];
+    shell = pkgs.zsh;
+    packages = with pkgs; [
+      tree
+    ];
+  };
+}