MCP: Support creating comments anchored to text in a document

Summary The MCP create_comment tool currently creates unanchored (document-level) comments. It would be valuable to support creating comments anchored to a specific text snippet, the same way users can highlight text and comment on it in the editor UI. Motivation MCP-powered AI agents are increasingly used for document review workflows — reviewing drafts, flagging issues, suggesting edits. Today, create_comment can only leave general comments on a document. This means an agent reviewing a document has no way to point at the specific sentence or paragraph it's commenting on, making its feedback harder to act on. Anchored comments would make MCP-based review workflows much more useful, since each comment would appear inline next to the relevant text, just like comments left by human reviewers through the editor. How anchored comments work today From reading the codebase, anchored comments are a two-part mechanism: 1. A comment mark (with attributes id, userId, …