useMessages

Subscribe to a scoped messages stream.

Contract:

• At the root (no target, or a static target that resolves to the root namespace) returns a handle whose .current delegates to 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 owning component teardown fires (and the registry closes the underlying server subscription when the last consumer leaves).
• A reactive target (getter form) re-binds the subscription on change. A getter that flips between root and scoped is supported: the root case short-circuits to the initial value because dynamic root delegation isn't meaningful — pass a static undefined/null target for root handles.

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