feat: add builder mode for centralized Nix builds

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>

internal/metrics/server.go

@@ -74,6 +74,11 @@ func (s *Server) Collector() *Collector {
 	return s.collector
 }
 
+// Registry returns the Prometheus registry.
+func (s *Server) Registry() *prometheus.Registry {
+	return s.registry
+}
+
 // ScrapeCh returns a channel that receives a signal each time the metrics endpoint is scraped.
 func (s *Server) ScrapeCh() <-chan struct{} {
 	return s.scrapeCh