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

.gitignore

@@ -8,3 +8,6 @@ env/
 .env
 
 **/.DS_Store
+
+# Installer ISO live WiFi (SSID/PSK); see docs/server-installer-usb.md
+nixos/installer-wifi.nix