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.

flake.nix

@@ -16,6 +16,13 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    # Declarative disk partitioning. Each Host declares its own layout; a Host
+    # that preserves an existing pool simply declares none.
+    disko = {
+      url = "github:nix-community/disko";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     # CachyOS kernel + binary cache. Deliberately NOT following our nixpkgs, so the
     # chaotic cache stays usable and the kernel is fetched rather than compiled.
     chaotic.url = "github:chaotic-cx/nyx/nyxpkgs-unstable";