feat(nixos): migrate system configuration

configs/nixos/hosts/pikachu/disk.nix

@@ -0,0 +1,32 @@
+{ ... }:
+# pikachu's install layout for disko: one NVMe boot disk with an EFI system partition and ext4 root.
+# The existing 8 TB ZFS mirror is imported by name and is never declared here.
+{
+  disko.devices.disk.main = {
+    type = "disk";
+    device = "/dev/nvme0n1";
+    content = {
+      type = "gpt";
+      partitions = {
+        ESP = {
+          size = "2G";
+          type = "EF00";
+          content = {
+            type = "filesystem";
+            format = "vfat";
+            mountpoint = "/boot";
+            mountOptions = [ "umask=0077" ];
+          };
+        };
+        root = {
+          size = "100%";
+          content = {
+            type = "filesystem";
+            format = "ext4";
+            mountpoint = "/";
+          };
+        };
+      };
+    };
+  };
+}