feat(engine): add workflow extension plugin seams

packages/plugin-sdk/src/index.ts

@@ -81,6 +81,36 @@ export type {
   PluginTraitContribution,
   PluginTraitHookDescriptor,
   PluginTraitFlags,
+  WorkflowExtensionContribution,
+  WorkflowExtensionMetadata,
+  WorkflowExtensionBaseContribution,
+  WorkflowColumnMetadataExtensionContribution,
+  WorkflowMovePolicyExtensionContribution,
+  WorkflowWorkEngineExtensionContribution,
+  WorkflowNodeHandlerExtensionContribution,
+  TaskVerdictProviderExtensionContribution,
+  AutoMergeFactProviderExtensionContribution,
+  WorkflowExtensionConfigField,
+  WorkflowExtensionConfigSchema,
+  WorkflowExtensionFallback,
+  WorkflowExtensionKind,
+  WorkflowMovePolicyDecision,
+  WorkflowMovePolicyInput,
+  WorkflowMovePolicyHandler,
+  WorkflowWorkEngineDispatchResult,
+  WorkflowWorkEngineInput,
+  WorkflowWorkEngineHandler,
+  WorkflowNodeExtensionResult,
+  WorkflowNodeHandlerInput,
+  WorkflowNodeExtensionHandler,
+  TaskVerdictStatus,
+  TaskVerdictProviderInput,
+  TaskVerdictProviderResult,
+  TaskVerdictProviderHandler,
+  AutoMergeRoute,
+  AutoMergeFactProviderInput,
+  AutoMergeFactProviderResult,
+  AutoMergeFactProviderHandler,
   PluginPromptSurface,
   PluginPromptContribution,
   PluginPromptContributions,
@@ -94,6 +124,16 @@ export type {
   FusionPlugin,
   PluginState,
   PluginInstallation,
+  BoardActionServices,
+  BoardActionTaskStore,
+  MoveBoardTaskInput,
+  UpdateBoardTaskInput,
+} from "@fusion/core";
+
+export {
+  WORKFLOW_EXTENSION_SCHEMA_VERSION,
+  workflowExtensionRegistryId,
+  createBoardActionServices,
 } from "@fusion/core";
 
 // ── Step-inversion IR types (type-only) ──────────────────────────────────────