feat(installer): enhance NixOS installer with SSH access and encryption password input

systems/iso/default.nix

@@ -24,11 +24,32 @@
 
   networking.hostName = "installer";
 
+  # On flake-built systems <nixpkgs> resolves through the flake registry,
+  # so nixos-install fails without these features enabled.
+  nix.settings.experimental-features = [
+    "flakes"
+    "nix-command"
+  ];
+
   environment.systemPackages = [
     outputs.packages.${pkgs.stdenv.hostPlatform.system}.installer-nixos
   ];
 
+  # Live-media only: sshd is already enabled by the installation profile,
+  # but ssh logins need a non-empty password.
+  users.users = {
+    nixos = {
+      password = "nixos";
+      initialHashedPassword = lib.mkForce null;
+    };
+    root = {
+      password = "nixos";
+      initialHashedPassword = lib.mkForce null;
+    };
+  };
+
   services.getty.helpLine = ''
     Run "sudo nixos-installer" to install NixOS onto a ZFS root pool.
+    SSH is enabled; the "nixos" and "root" passwords are "nixos".
   '';
 }