ax(mining): replace prose comment on ServiceStartup with usage example

AX Principle 2 — comments show HOW with real values, not describe WHAT
the function signature already says.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/service.go

@@ -572,8 +572,7 @@ func (s *Service) Stop() {
 	}
 }
 
-// ServiceStartup initializes the router and starts the HTTP server.
-// For embedding without a standalone server, use InitRouter() instead.
+// svc.ServiceStartup(ctx) // blocks until ctx is cancelled; use InitRouter() to embed without a server
 func (s *Service) ServiceStartup(ctx context.Context) error {
 	s.InitRouter()
 	s.Server.Handler = s.Router