Add typed prompt sources and backend inheritance

backends.go

@@ -19,10 +19,13 @@ const BackendLocal = "local"
 // explicit zero or blank values. A nil ExtraParams map is omitted; a non-nil
 // empty map supplies an empty mapping. A nil SmokeTests pointer is omitted; a
 // non-nil pointer supplies a smoke_tests mapping. Use raw ConfigKit Mapping
-// inputs for explicit null. BackendConfig has no stable JSON representation.
+// inputs for explicit null. Extends names a sibling backend in the same list.
+// BackendConfig has no stable JSON representation.
 type BackendConfig struct {
 	// ID is the local backend identity within its resource list.
 	ID string
+	// Extends names a sibling backend to inherit through ConfigKit.
+	Extends *string
 	// Endpoint is the required absolute HTTP(S) base URL after all layers merge.
 	Endpoint *string
 	// APIKeyEnv optionally names an environment variable, not a credential.
@@ -84,6 +87,9 @@ func BackendSource(label string, backends []BackendConfig) configkit.Source {
 	definitions := make([]any, 0, len(backends))
 	for _, config := range backends {
 		fields := map[string]any{"id": config.ID}
+		if config.Extends != nil {
+			fields["extends"] = map[string]any{"id": *config.Extends}
+		}
 		if config.Endpoint != nil {
 			fields["endpoint"] = *config.Endpoint
 		}