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.

flake.nix

@@ -31,5 +31,10 @@
 			username = "marvin";
 			homePath = ./hosts/hivebrain/home.nix;
 		};
+		nixosConfigurations.nikita-nixos = mkHost {
+			hostPath = ./hosts/nikita-nixos;
+			username = "nikita";
+			homePath = ./hosts/nikita-nixos/home.nix;
+		};
 	};
 }