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-runtime/src/build-context.ts

@@ -54,6 +54,7 @@ async function threadFromStartHead<M extends RoleMeta>(
   return {
     threadId: "",
     depth: p.depth,
+    bundleHash: p.hash,
     start: {
       role: START,
       content: prompt,
@@ -113,6 +114,7 @@ async function threadFromStateHead<M extends RoleMeta>(
   return {
     threadId: "",
     depth: sp.depth,
+    bundleHash: sp.hash,
     start: {
       role: START,
       content: prompt,

packages/workflow-runtime/src/create-workflow.ts

@@ -6,6 +6,7 @@ import {
   type AgentBinding,
   type AgentContext,
   type AgentFn,
+  type AgentFnResult,
   END,
   type ModeratorContext,
   type RoleDefinition,
@@ -49,6 +50,16 @@ function mergeUniqueHashes(a: readonly string[], b: readonly string[]): string[]
   return out;
 }
 
+function normalizeAgentResult(result: AgentFnResult): {
+  output: string;
+  childThread: string | null;
+} {
+  if (typeof result === "string") {
+    return { output: result, childThread: null };
+  }
+  return result;
+}
+
 function agentForRole(binding: AgentBinding, roleName: string): AgentFn {
   const overrides = binding.overrides;
   const overrideFn: AgentFn | undefined =
@@ -86,9 +97,9 @@ async function advanceOneRound<M extends RoleMeta>(
   };
 
   const agent = agentForRole(binding, next);
-  const raw = await agent(agentCtx as unknown as AgentContext);
+  const agentResult = normalizeAgentResult(await agent(agentCtx as unknown as AgentContext));
 
-  const agentContentHash = await putContentNodeWithRefs(runtime.cas, raw, []);
+  const agentContentHash = await putContentNodeWithRefs(runtime.cas, agentResult.output, []);
 
   const extracted = await runtime.extract(
     roleDef.schema as z.ZodType<Record<string, unknown>>,
@@ -122,6 +133,7 @@ async function advanceOneRound<M extends RoleMeta>(
       contentHash: step.contentHash,
       meta: step.meta,
       refs: step.refs,
+      childThread: agentResult.childThread,
     },
     step,
   };

packages/workflow-runtime/src/index.ts

@@ -5,6 +5,7 @@ export type {
   AgentBinding,
   AgentContext,
   AgentFn,
+  AgentFnResult,
   CasStore,
   ExtractFn,
   ExtractResult,

packages/workflow-runtime/src/types.ts

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