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-util-agent/__tests__/build-agent-prompt.test.ts

@@ -17,6 +17,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("fix the bug"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       steps: [],
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: START, systemPrompt: "You are an agent." },
@@ -33,6 +34,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("user task"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "coder", systemPrompt: "Be helpful." },
       steps: [
@@ -61,6 +63,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("first message full: task content here"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "coder", systemPrompt: "System." },
       steps: [
@@ -99,6 +102,7 @@ describe("buildAgentPrompt", () => {
     const ctx: AgentContext = {
       start: startTask("start"),
       depth: 0,
+      bundleHash: "TESTHASH00001",
       threadId: "01TEST000000000000000000TR",
       currentRole: { name: "c", systemPrompt: "S" },
       steps: [