migrated all languages to devshell

modules/dev/nix.nix

@@ -4,16 +4,34 @@
   pkgs,
   ...
 }:
+let
+  packages = builtins.attrValues {
+    inherit (pkgs)
+      nixfmt-rfc-style # formatting
+      cachix # binary cache management
+      nixd # language server for Nix
+      statix # linter for Nix expressions
+      ;
+  };
+in
 {
-  options.my.dev.nix.enable = lib.mkEnableOption "enable";
+  options = {
+    my.dev.nix.enable = lib.mkEnableOption "Install Nix tooling globally";
+    devShells.nix = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.mkShell {
+        inherit packages;
+        name = "nix-dev-shell";
+        shellHook = ''
+          echo "❄️  Nix dev environment"
+        '';
+      };
+      description = "Nix/NixOS development shell with formatter, linter, LSP, and Cachix";
+    };
+  };
   config = lib.mkIf config.my.dev.nix.enable {
-    users.users.jawz.packages = builtins.attrValues {
-      inherit (pkgs)
-        nixfmt-rfc-style # formating
-        cachix # why spend time compiling?
-        nixd # language server
-        statix # linter
-        ;
+    users.users.jawz = {
+      inherit packages;
     };
   };
 }