feat: Phase 3 — agent observability for Merkle call stack

- StartStep gains parentState: string | null (from StartNodePayload)
- buildAgentPrompt injects Parent Context section when parentState is set
- CLI thread show outputs parentState (top-level) and childThread (per step)
- 2 new prompt tests + thread show assertion updates

Refs #197, #194

小橘 🍊（NEKO Team）

packages/workflow-runtime/src/build-context.ts

@@ -60,6 +60,7 @@ async function threadFromStartHead<M extends RoleMeta>(
       content: prompt,
       meta: {},
       timestamp: 0,
+      parentState: p.parentState,
     },
     steps: [],
   };
@@ -120,6 +121,7 @@ async function threadFromStateHead<M extends RoleMeta>(
       content: prompt,
       meta: {},
       timestamp: firstTs,
+      parentState: sp.parentState,
     },
     steps,
   };