fix(config): reject a hidden YAML document and drop Go types from errors

internal/config/bytesize.go

@@ -33,7 +33,7 @@ func (size *byteSize) Set(value string) error {
 // UnmarshalYAML reads the raw scalar so a size parses the same way here as on the command line.
 func (size *byteSize) UnmarshalYAML(node *yaml.Node) error {
 	if node.Kind != yaml.ScalarNode {
-		return fmt.Errorf("expected a size, but got %s", node.Tag)
+		return fmt.Errorf("expected a size, but got %s", yamlNodeName(node))
 	}
 
 	return size.Set(node.Value)