Declare neogaia's encrypted disk with disko

Add disko as a flake input (following nixpkgs) and wire its NixOS module
into the host-builder, so any Host can declare a disk layout while the
layout itself stays a per-Host concern.

neogaia's layout (hosts/neogaia/disk.nix): one GPT NVMe disk with a
systemd-boot EFI system partition and a LUKS container holding btrfs with
@root/@home/@nix subvolumes. The initrd prompts for the passphrase on a
normal boot. Swap is RAM-backed zram, so there is no on-disk swap
partition; zramSwap is enabled directly for now. The placeholder
fileSystems are removed since disko now derives them.

lib/default.nix

@@ -54,6 +54,7 @@ let
         ++ [
           inputs.home-manager.nixosModules.home-manager
           inputs.chaotic.nixosModules.default
+          inputs.disko.nixosModules.disko
           (self + "/system")
           (self + "/hosts/${hostName}")
           { networking.hostName = hostName; }