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).

lib/keys.nix

@@ -9,6 +9,13 @@
     ai-worker = {
       main = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAXeGtPPcsP2IYRQNvII41NVWhJsarEk8c4qxs/a5sXf";
     };
+
+    builder = {
+      # Same key on both hosts for bidirectional remote building.
+      # Generate with: ssh-keygen -t ed25519 -f /etc/ssh/builder_key -N ""
+      # Replace the placeholder below with the public key (builder_key.pub).
+      main = "PLACEHOLDER_ADD_BUILDER_PUBKEY_HERE";
+    };
   };
 
   hosts = {