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.lock

@@ -21,6 +21,26 @@
         "type": "github"
       }
     },
+    "disko": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1781152676,
+        "narHash": "sha256-RxWs5ND31KzTG7wvMM+PMfUjyNpmIEr999lqNARaM5o=",
+        "owner": "nix-community",
+        "repo": "disko",
+        "rev": "ff8702b4de27f72b4c78573dfb89ec74e36abdf1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "disko",
+        "type": "github"
+      }
+    },
     "flake-schemas": {
       "locked": {
         "lastModified": 1780327564,
@@ -143,6 +163,7 @@
     "root": {
       "inputs": {
         "chaotic": "chaotic",
+        "disko": "disko",
         "home-manager": "home-manager_2",
         "nixpkgs": "nixpkgs_2",
         "nixpkgs-stable": "nixpkgs-stable",