feat(engine): WorkflowGraphTaskRunner — interpreter drives a task's lifecycle (CU-U2)

Loads a task's selected workflow, runs the graph with injected legacy seams
(execute/review/merge) and a custom-node runner, and maps the terminal outcome
to completed/failed/fell-back. Any interpreter-level error falls back so the
caller can run the legacy pipeline — a task is never stranded. Covered with
fake seams: lifecycle ordering, failure routing, gate blocking, fallback
reasons, diagnostics isolation. Includes the interpreter-cutover plan doc.

packages/engine/src/index.ts

@@ -24,9 +24,17 @@ export {
 export {
   createDefaultNodeHandlers,
   createNoopLegacySeams,
+  type WorkflowCustomNodeRunner,
   type WorkflowLegacySeams,
   type WorkflowSeamName,
 } from "./workflow-node-handlers.js";
+export {
+  WorkflowGraphTaskRunner,
+  type WorkflowGraphRunDisposition,
+  type WorkflowGraphRunnerStore,
+  type WorkflowGraphTaskRunResult,
+  type WorkflowGraphTaskRunnerDeps,
+} from "./workflow-graph-task-runner.js";
 export { collectTaskEvaluationEvidence } from "./evaluator-evidence.js";
 export { Scheduler, type SchedulerOptions } from "./scheduler.js";
 export { MeshLeaseManager, type MeshLeaseManagerOptions, type LeaseRecoveryContext } from "./mesh-lease-manager.js";