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-protocol/__tests__/moderator-table.test.ts

@@ -21,6 +21,7 @@ function makeCtx(roles: (keyof TestMeta & string)[]): ModeratorContext<TestMeta>
   return {
     threadId: "test-thread",
     depth: 0,
+    bundleHash: "TESTHASH00001",
     start: {
       role: START,
       content: "test",

packages/workflow-protocol/src/index.ts

@@ -13,6 +13,7 @@ export type {
   AgentBinding,
   AgentContext,
   AgentFn,
+  AgentFnResult,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-protocol/src/types.ts

@@ -41,6 +41,7 @@ export type RoleOutput = {
   contentHash: string;
   meta: Record<string, unknown>;
   refs: string[];
+  childThread: string | null;
 };
 
 export type StartStep = {
@@ -63,6 +64,7 @@ export type RoleStep<M extends RoleMeta> = {
 export type ThreadContext<M extends RoleMeta = RoleMeta> = {
   threadId: string;
   depth: number;
+  bundleHash: string;
   start: StartStep;
   steps: RoleStep<M>[];
 };
@@ -127,7 +129,9 @@ export type ExtractFn = <T extends Record<string, unknown>>(
   contentHash: string,
 ) => Promise<ExtractResult<T>>;
 
-export type AgentFn = (ctx: AgentContext) => Promise<string>;
+export type AgentFnResult = string | { output: string; childThread: string | null };
+
+export type AgentFn = (ctx: AgentContext) => Promise<AgentFnResult>;
 
 export type AgentBinding = {
   agent: AgentFn;