Show a headings sidebar in the built-in hybrid Markdown editor

- I'd like a sidebar showing the document's headings in the built-in hybrid Markdown editor.
- Agents are creating more Markdown files for plans and reports. These documents can get long, making it difficult to find a particular section by scrolling.
- A headings list alongside the document would make its structure easy to scan. Clicking a heading should jump to that section without leaving the hybrid editor.