Add a new "builder" capability to trigger Nix builds on a dedicated build host via NATS messaging. This allows pre-building NixOS configurations before deployment. New components: - Builder mode: subscribes to build.<repo>.* subjects, executes nix build - Build CLI command: triggers builds with progress tracking - MCP build tool: available with --enable-builds flag - Builder metrics: tracks build success/failure per repo and host - NixOS module: services.homelab-deploy.builder The builder uses a YAML config file to define allowed repositories with their URLs and default branches. Builds can target all hosts or specific hosts, with real-time progress updates. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>