feat(#194): Phase 2 — Engine layer Merkle call stack wiring

- Protocol: AgentFnResult = string | { output, childThread }, RoleOutput.childThread,
  ThreadContext.bundleHash for parent state lookup
- Runtime: create-workflow normalizes AgentFnResult, propagates childThread in RoleOutput
- Engine: ExecuteThreadOptions.parentStateHash, appendStateForStep writes childThread,
  putStartNode uses parentStateHash
- workflowAsAgent: reads parent head state from threads.json, passes parentStateHash
  to child, returns { output, childThread: rootHash }
- Integration test: 4 cases verifying bidirectional Merkle links (306 lines)

Phase 2 of #194 (Merkle Call Stack). Closes #196.

小橘 <xiaoju@shazhou.work>

packages/workflow-template-develop/__tests__/develop-template.test.ts

@@ -26,6 +26,7 @@ function makeCtx(steps: ModeratorContext<DevelopMeta>["steps"]): ModeratorContex
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: makeStart(),
     steps,
   };