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.

hosts/nikita-nixos/default.nix

@@ -0,0 +1,25 @@
+{ ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+
+    # shared
+    ../../system/boot.nix
+    ../../system/networking.nix
+    ../../system/locale.nix
+    ../../system/fonts.nix
+    ../../system/nix.nix
+    ../../system/ssh.nix
+    ../../system/programs.nix
+    ../../system/packages.nix
+
+    # nikita-only
+    ../../system/plasma.nix
+    ../../system/display-managers/sddm.nix
+    ../../system/users/nikita.nix
+  ];
+
+  networking.hostName = "nikita-nixos";
+  system.stateVersion = "26.05";
+}