Feature Request: Support @user mentions and integration mentions when creating/updating documents via MCP

Summary When creating or updating documents through the built-in MCP server, there is no way to insert native mentions — neither @user mentions nor integration mentions (GitLab issues, GitHub PRs, Linear issues, etc.). All mention-like content is rendered as plain text or simple links, never as interactive ProseMirror mention nodes. Steps to reproduce Test 1: @user mention 1. Connect to Outline's built-in MCP server from an AI assistant (e.g. Claude via claude.ai) 2. Use the create_document tool with markdown content: 3. Open the created document in Outline 4. Result: @AI is rendered as plain text — not clickable, no notification sent. Test 2: GitLab issue mention (URL brute) 1. Use the create_document tool with: 2. Result: The URL is rendered as plain text. In the editor, pasting this same URL would trigger a popup offering "Keep as link", "Mention", or "Embed" — but via MCP, there is no way to select "Mention&…