Diagnose: no token chunks when streaming a parent graph that wraps an agent

# Symptom

You wrapped a `create_agent` result as a node in a parent StateGraph. Streaming the parent with `stream_mode="messages"` shows no token chunks from the inner agent's model calls.

## Confirm

Check the structure: `create_agent` returns a compiled graph, so using it as a node makes it a subgraph. Parent-level message streaming does not cross subgraph boundaries by default.

## Cause

Token chunks from nested graphs are suppressed unless you opt in. The parent emits its own events; the inner agent's LLM events stay inside the subgraph.

## Fix

```python
for chunk in parent_graph.stream(
    {...},
    stream_mode="messages",
    subgraphs=True,
):
    ...
```

## Verify

Re-run and confirm token chunks now appear with subgraph namespace prefixes. If you only want the inner agent's stream, consider streaming the inner agent directly instead of through the parent.