inline common-darwin into poppy (only one darwin host for now)

- merged common-darwin/default.nix content into hosts/poppy/default.nix
- removed hosts/common-darwin/
- removed ./hosts/common-darwin import from mkDarwin in flake.nix

linux hosts/common/ stays — already 2 linux hosts (muscari + rosemary)

flake.nix

@@ -51,7 +51,6 @@
         nix-darwin.lib.darwinSystem {
           system = "aarch64-darwin";
           modules = [
-            ./hosts/common-darwin
             ./hosts/${hostname}
             home-manager.darwinModules.home-manager
             {