feat(dev): deploy claude-code config via systemd instead of home-manager

Home-manager fails on the dev microVM because nix-env --set needs
writable nix state dirs, but the microVM shares /nix/store read-only
via virtiofs.

Extract shared claude-code settings into settings.nix, add a NixOS
module (nixos.nix) that deploys the same files via a systemd oneshot
service with RequiresMountsFor to handle virtiofs mount ordering.
The nb host continues using home-manager unchanged.

hosts/nb/users/dominik.nix

@@ -188,7 +188,9 @@ in
   };
 
   home-manager.users.dominik = { lib, pkgs, ... }: {
-    # imports = [ "${impermanence}/home-manager.nix" ];
+    imports = [
+      ../utils/home-manager/claude-code
+    ];
     /* The home.stateVersion option does not have a default and must be set */
     home.stateVersion = "25.05";
     home.enableNixpkgsReleaseCheck = false;