feat: bidirectional remote builders — server <> uConsole

Add reusable remote-builder NixOS module and builder system user.

Server (lazyworkhorse): dispatches aarch64-linux builds to uConsole
uConsole: dispatches x86_64-linux builds to server

The builder user uses the same SSH keypair on both hosts for
symmetric remote building. Generate the key with:
  ssh-keygen -t ed25519 -f /etc/ssh/builder_key -N ""
Add the public key to lib/keys.nix (replace PLACEHOLDER).

flake.nix

@@ -69,9 +69,11 @@
               ./modules/nixos/services/open_code_server.nix
               ./modules/nixos/services/ollama_init_custom_models.nix
               ./modules/nixos/services/openclaw_node.nix
+              ./modules/nixos/services/remote-builder.nix
               ./modules/nixos/security/ai-worker-restricted.nix
               ./users/gortium.nix
               ./users/ai-worker.nix
+              ./users/builder.nix
             ];
           };
 
@@ -100,7 +102,9 @@
               }
               nixos-raspberrypi.nixosModules.raspberry-pi-5.base
               nixos-uconsole.nixosModules.uconsole-cm5
+              ./modules/nixos/services/remote-builder.nix
               ./hosts/uConsole/configuration.nix
+              ./users/builder.nix
               ./hosts/uConsole/hardware-configuration.nix
             ];
           };