feat(nixos): migrate system configuration

configs/nixos/hosts/neogaia/default.nix

@@ -0,0 +1,53 @@
+{
+  inputs,
+  pkgs,
+  ...
+}:
+# neogaia — Dell XPS 13 9380 laptop.
+# Disk layout is in ./disk.nix.
+# `fileSystems` are derived from it, none declared here.
+{
+  imports = [
+    inputs.nixos-hardware.nixosModules.dell-xps-13-9380
+    ./hardware-configuration.nix
+    ./disk.nix
+  ];
+
+  system.stateVersion = "26.05";
+
+  # systemd-boot on the EFI system partition.
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  boot.kernelPackages = pkgs.linuxPackages_cachyos;
+
+  # Redistributable firmware for the QCA6174 wifi (ath10k blobs).
+  # Intel microcode updates follow from this, so none is declared here.
+  hardware.enableRedistributableFirmware = true;
+
+  # RAM-backed swap, no on-disk swap partition.
+  zramSwap.enable = true;
+
+  # So wifi can be joined from the console.
+  networking.networkmanager.enable = true;
+
+  registry.address.neogaia.self = "10.23.50.146";
+
+  # So setup can be driven over the network.
+  # The matching host public keys sit beside this file in plaintext, since
+  # publishing them is their purpose.
+  modules.network.ssh.enable = true;
+  modules.network.ssh.hostKeys.sopsFile = ../../secrets/neogaia.yaml;
+  modules.network.ssh.userKey.sopsFile = ../../secrets/neogaia.yaml;
+
+  modules.shell.herdr.enable = true;
+  modules.agents.tools.gitea-axi.enable = true;
+  modules.agents.pi.enable = true;
+
+  modules.desktop.enable = true;
+  modules.desktop.apps.obsidian.enable = true;
+  modules.desktop.apps.steam.enable = true;
+
+  time.timeZone = "America/New_York";
+  i18n.defaultLocale = "en_GB.UTF-8";
+}