Convert HTML wrappers, headings, lists and picture in the pull request reader #1008

The HTML pass introduced in #1000 converts a fixed allow-list; a block holding any other tag stays source, all or nothing. Tags PR descriptions use routinely and github.com renders, still shown as source:

• <p>, <div>, <span> — wrappers with no effect of their own (<p align="center"><img …></p> is the common shape for a centred screenshot); an allow-list entry that contributes only its children.
• <h1>–<h6> written as HTML — a HeadingBlock of the matching level.
• <ul>/<ol>/<li> written as HTML — a ListBlock.
• <picture> with <source> and a fallback <img> — the fallback image's label.
• A <summary> in a separate block from its <details> (a blank line between them); today the summary block is rejected and shows inside the folded section as source, with the header reading Details.

Each maps onto a Markdig node MarkView already renders, so no renderer work — only the block rule's tag classes and the builder. Keep the depth budget and the degrade rule from docs/superpowers/specs/2026-09-17-ai2889-github-markdown-pr-reader-design.md.