feat(user:shell): add system modules for zsh/bash user shell configuration

system/modules/user/shell/default.nix

@@ -1,40 +1,19 @@
-{ lib, config, ... }:
+{ lib, ... }:
 
 let
-  cfg = config.systemModules.user.shell;
+  inherit (lib) types mkOption;
 in
 
 {
   imports = [
     ./fish
-    # ./bash
-    # ./zsh
+    ./bash
+    ./zsh
   ];
 
-  options.systemModules.user.shell = {
-    fish = {
-      enable = lib.mkEnableOption "Enable fish as the user shell";
-    };
-    zsh = {
-      enable = lib.mkEnableOption "Enable zsh as the user shell";
-    };
-    bash = {
-      enable = lib.mkEnableOption "Enable bash as the user shell";
-    };
-
-  };
-
-   config = { 
-    assertions = [
-      {
-        assertion = 
-          (lib.length (lib.filter (x: x) [
-            cfg.fish.enable or false
-            cfg.zsh.enable or false
-            cfg.bash.enable or false
-          ]) <= 1); 
-        message = "Only one user shell can be active in the configuration";
-      }
-    ];
+  options.systemModules.user.shell = mkOption {
+    type = types.enum ["fish" "zsh" "bash"];
+    default = "zsh";
+    description = "The user shell to use. Select from 'zsh' 'bash' 'fish'";
   };
 }