Merge feat/remote-builders into uconsole-cm5-incremental

Bidirectional remote building + rename uconsole-cm5 -> uConsole:
- Rename all uconsole-cm5 refs to uConsole (nixosSystem key, paths, module)
- Replace single-machine remote-builder module with multi-machine config
- Enable remote builder on lazyworkhorse -> uConsole (aarch64)
- Enable remote builder on uConsole -> lazyworkhorse (x86_64)
- Add builder system user + SSH key via agenix-rekey (secrets/builder_key.age)
- Add builder_key + gortium_password age secrets to all hosts
- Fix pre-existing bugs: missing modules in lazyworkhorse/cyt-pi,
  SSH config structure (hostKeys outside settings), cyt-pi inline user
- Remove obsolete openclaw-node service + module + age secrets

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 = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIByG79NM/rk13q3MTow9Ezb8pe3RWoQ76Mv8atASp3Ri";
+    };
   };
 
   hosts = {