refactor: Make --context cli flag a global option (#174)

* Make `--context` flag a global option and remove duplication

* Remove unused dns showOptions struct

* First pass of refactoring "active context" determination and validation

* Undo some refactoring in cli.go to keep diff more focused

cmd/uncloud/build.go

@@ -74,8 +74,6 @@ to cluster machines or --push-registry to upload them to external registries.`,
 			"Use --machine to specify which machines. (default is all machines)")
 	cmd.Flags().BoolVar(&opts.PushRegistry, "push-registry", false,
 		"Upload the built images to external registries (e.g., Docker Hub) after building.")
-	cmd.Flags().StringVarP(&opts.Context, "context", "c", "",
-		"Name of the cluster context. (default is the current context)")
 
 	return cmd
 }