Add citadel to flake.nix

flake.nix

@@ -130,6 +130,16 @@
             }
           ];
         };
+        citadel = nixpkgs.lib.nixosSystem {
+          inherit specialArgs;
+          modules = [
+            ./hosts/citadel
+            home-manager.nixosModules.home-manager
+            {
+              home-manager.extraSpecialArgs = specialArgs;
+            }
+          ];
+        };
       };
     };
 }