Use one conversation rail with two message scopes, rather than separate Chat and Comments products:

Document thread: messages attached to the current document or an exact block.

General chat: messages belonging to the shared space/session, independent of the open document.

Show scope on every message: General, This document, or Selected block.

Use one composer with a scope selector and the same people, agents, avatars, mentions, notifications, and reply threading.

Let users promote a chat message into a comment or capture an exchange as a document.

Preserve comments as durable, signed HM resources; chat can remain conversational, but should reference those same resources rather than duplicate them.

When navigating documents, the rail filters to relevant messages while still offering “All conversation.”

This directly resolves the noted ambiguity between agent conversations and document comments: one UX and message model, with explicit context and different persistence semantics.