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-solve-issue/__tests__/solve-issue-template.test.ts

@@ -113,6 +113,7 @@ function makeCtx(
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: makeStart(),
     steps,
   };
@@ -178,6 +179,7 @@ function makeThread(prompt: string) {
   return {
     threadId: "01TEST000000000000000000TR",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: {
       role: START,
       content: prompt,