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/disko-server.nix

@@ -0,0 +1,39 @@
+# Declarative disk layout for server installs via disko-install.
+# Device is injected at install time: disko-install --disk main /dev/sda
+# LUKS passphrase is prompted interactively (no keyFile).
+{
+  disko.devices = {
+    disk.main = {
+      type = "disk";
+      content = {
+        type = "gpt";
+        partitions = {
+          ESP = {
+            size = "512M";
+            type = "EF00";
+            content = {
+              type = "filesystem";
+              format = "vfat";
+              mountpoint = "/boot";
+              mountOptions = [ "fmask=0022" "dmask=0022" ];
+            };
+          };
+          luks = {
+            size = "100%";
+            content = {
+              type = "luks";
+              name = "crypted";
+              settings.allowDiscards = true;
+              # No keyFile/passwordFile => interactive passphrase at install
+              content = {
+                type = "filesystem";
+                format = "ext4";
+                mountpoint = "/";
+              };
+            };
+          };
+        };
+      };
+    };
+  };
+}