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.