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.
