Enable shared configuration version checks

internal/configuration/load.go

@@ -17,6 +17,7 @@ import (
 )
 
 type document struct {
+	Version  *string                  `yaml:"version"`
 	Backends map[string]backendConfig `yaml:"backends"`
 	Profiles map[string]profileConfig `yaml:"profiles"`
 	Prompts  map[string]promptConfig  `yaml:"prompts"`
@@ -152,8 +153,13 @@ func load(main configkit.Input, underlays []configkit.Underlay) (*loaded, error)
 		promptMetadata: make(map[string][]promptInputPresence),
 	}
 	result, err := configkit.LoadLayered(main, configkit.LayeredOptions[document]{
-		Underlays: underlays,
+		Underlays:         underlays,
+		SourceVersionMode: configkit.SourceVersionModeIfPresent,
 		Definition: configkit.Definition[document]{
+			Version: &configkit.VersionRule{
+				Path:  configkit.Keys("version"),
+				Check: checkConfigurationVersion,
+			},
 			Secret: secretProviderPath,
 			Resolve: func(config *document, resolution *configkit.Resolution) error {
 				captureFields(config.Backends, bases.backends, backendFields, resolution, state.backends)
@@ -186,6 +192,16 @@ func load(main configkit.Input, underlays []configkit.Underlay) (*loaded, error)
 	return state, nil
 }
 
+func checkConfigurationVersion(version configkit.Version, validation *configkit.Validation) error {
+	if !version.Present {
+		return nil
+	}
+	if version.Null || version.Text != "1" {
+		validation.Error(configkit.Keys("version"), "unsupported configuration specification version")
+	}
+	return nil
+}
+
 type promptInputPresence struct {
 	contentType fieldState
 	description fieldState