API: Support creating anchored comments programmatically

Use case I'm using the API to have an AI assistant (Claude) review documents and leave inline comments with feedback — similar to how a human reviewer would highlight a passage and add a note. A key requirement is that the AI can suggest changes without modifying the document itself — anchored comments are the ideal mechanism for this, since I can review each suggestion and decide whether to accept it. Currently, all API-created comments land as unanchored document-level comments with no connection to the specific text they refer to, which significantly reduces their usefulness for this workflow. Below is optional to read - Claude's proposals are below (I don't really understand them) 1. Expose ProseMirror JSON via API — Add a format=prosemirror option to documents.export (or a new documents.prosemirror endpoint) that returns the full document node tree including mark IDs. API consumers could then extract the relevant anchorId for a given text selection and use i…