Allow MCP to control the editor/convert pages

For markdown-source pages at least, expose a tool that allows agents to convert between editors and create pages using a specified editor. Hopefully this is a straightforward change at least for markdown pages. Alternatively, an MCP setting that supports setting the default page type.

This allows non-technical users to edit pages both using MCP and the visual editor without having to see markdown source directly or understand how conversion works. It's a small thing that can make a disproportionately large difference for org buy-in.