feat: server-side remote builder config (lazyworkhorse dispatches aarch64 to uConsole)

Server only — uConsole-side config will be added in uconsole-cm5-incremental branch.
Includes reusable module, builder system user, and SSH key placeholder.

users/builder.nix

@@ -0,0 +1,13 @@
+{ config, lib, pkgs, keys, ... }: {
+  users.users.builder = {
+    isSystemUser = true;
+    group = "builder";
+    home = "/var/empty";
+    createHome = false;
+    shell = pkgs.nologin;
+    openssh.authorizedKeys.keys = with keys; [
+      users.builder.main
+    ];
+  };
+  users.groups.builder = {};
+}