feat(remote-build): Add john-endesktop as builder machine

- Enable enableBuilder role on john-endesktop
- Add john-endesktop to nix-book's builder list (maxJobs=1, speedFactor=1)
- Document SSH setup process for new clients in remote-build role

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

machines/john-endesktop/configuration.nix

@@ -104,6 +104,9 @@ with lib;
   # User configuration
   roles.users.enable = true;
 
+  # Enable as remote builder (similar to zix790prors)
+  roles.remote-build.enableBuilder = true;
+
   # k3s agent configuration
   roles.k3s-node = {
     enable = true;