Support collapsible sections via `<details>` and `<summary>`

[kevinmarr]

Should just follow the GitHub syntax since this is what agents are used to.

It should support:

• Render Markdown Inside: Markdown lists, headers, etc should render correctly inside the collapsible section.
• Expand by Default: Add the open attribute directly inside the tag to force the section to load open: <details open>.

As for styling, unlike Notion I think it would be best if the expand/collapse caret did NOT push the header over; ideally collapsible headers and non-collapsible headers maintain their left-alignment.