What's verified:
  - nix flake check passes for both hivebrain and nikita-nixos
  - nixos-rebuild dry-build --flake .#hivebrain succeeds (your config is unchanged in behavior)
  - nixos-rebuild dry-build --flake .#nikita-nixos succeeds (Plasma 6 + SDDM derivation resolves)
  - main is untouched

  What's still TODO before you can install nikita-nixos:
  - Replace hosts/nikita-nixos/hardware-configuration.nix (placeholder with PLACEHOLDER-ROOT label) with
  the real output of nixos-generate-config --root /mnt on his machine.
  - Replace the placeholder git identity in hosts/nikita-nixos/home.nix (currently Nikita /
  nikita@example.com).
  - Set Nikita's password with passwd after first boot — nothing is baked in.