useMessages

Subscribe to a scoped messages stream.

Contract:

• At the root (no target) this returns stream.messages — the always-on root projection; no extra subscription is opened.
• For any non-root namespace, mount triggers a ref-counted messages subscription scoped to that namespace. The subscription is released automatically when the calling scope disappears (and the registry closes the underlying server subscription when the last consumer leaves).

Messages are always BaseMessage class instances from @langchain/core/messages.