darwin compatibility logic & darwin mcp

parts/devshells.nix

@@ -1,16 +1,23 @@
 { inputs, ... }:
 {
-  perSystem = _: {
-    devShells =
-      let
-        hostShells = inputs.self.nixosConfigurations.emacs.config.devShells;
-      in
-      inputs.self.lib.langList
-      |> builtins.filter (name: hostShells ? ${name})
-      |> map (name: {
-        inherit name;
-        value = hostShells.${name};
-      })
-      |> builtins.listToAttrs;
-  };
+  perSystem =
+    { pkgs, ... }:
+    {
+      devShells =
+        if pkgs.stdenv.isLinux then
+          let
+            hostShells = inputs.self.nixosConfigurations.emacs.config.devShells;
+          in
+          inputs.self.lib.langList
+          |> builtins.filter (name: hostShells ? ${name})
+          |> map (name: {
+            inherit name;
+            value = hostShells.${name};
+          })
+          |> builtins.listToAttrs
+        else
+          {
+            mcp = (import ../modules/dev/mcp/common.nix { inherit pkgs inputs; }).devShell;
+          };
+    };
 }