Add NixOS server installer USB (disko LUKS + WiFi, hostname prompt)

- disko-server.nix: LUKS + ESP + ext4 root layout for disko-install
- server-install: minimal NixOS config for new servers (hostname/WiFi via --system-config)
- installer-iso: custom minimal ISO with iwlwifi; build with nix build .#installer-iso
- scripts/nixos-server-install.sh: prompt hostname/disk, run disko-install
- docs/server-installer-usb.md: build, write USB, optional live/installed WiFi
- .gitignore: nixos/installer-wifi.nix; AGENTS.md + README.md notes

Made-with: Cursor

nixos/installer-iso.nix

@@ -0,0 +1,14 @@
+# Custom minimal NixOS installer ISO for server installs (disko-install).
+# Optional: add nixos/installer-wifi.nix (gitignored) to the flake modules to
+# preconfigure live-system WiFi so the installer can reach the network.
+{ config, pkgs, modulesPath, ... }:
+
+{
+  imports = [
+    (modulesPath + "/installer/cd-dvd/installation-cd-minimal.nix")
+  ];
+
+  # Kernel modules for typical server WiFi (Intel). Add others if needed for your hardware.
+  boot.kernelModules = [ "iwlwifi" ];
+  boot.extraModulePackages = [ ];
+}