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/hardware-configuration.nix

@@ -0,0 +1,24 @@
+{ lib, modulesPath, ... }:
+
+# Placeholder hardware-configuration. Replace with the file generated on
+# Nikita's actual machine via `nixos-generate-config --root /mnt` before
+# building or installing.
+{
+  imports = [
+    (modulesPath + "/installer/scan/not-detected.nix")
+  ];
+
+  boot.initrd.availableKernelModules = [ ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ ];
+  boot.extraModulePackages = [ ];
+
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/PLACEHOLDER-ROOT";
+    fsType = "ext4";
+  };
+
+  swapDevices = [ ];
+
+  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
+}