chore: freeze EventEnvelope v0 byte contract

vm-copilot/vm_copilot_agent.py

@@ -113,7 +113,7 @@ def post_event(kind: str, node_id: str, body: dict) -> Optional[dict]:
         "kind": kind,
         "node_id": node_id,
         "author": AUTHOR,
-        "body": body,
+        "payload": body,
     }
     try:
         resp = requests.post(
@@ -122,7 +122,7 @@ def post_event(kind: str, node_id: str, body: dict) -> Optional[dict]:
             headers={"Content-Type": "application/json"},
             timeout=10,
         )
-        if resp.status_code == 200:
+        if resp.status_code in (200, 201):
             result = resp.json()
             log.info(f"Posted {kind} event: {result.get('id', 'unknown')}")
             return result