ProseMirror schema (headings 1-4, lists incl. task lists, blockquote,
code block, tables via prosemirror-tables, images, hard breaks; bold/
italic/code/strikethrough/link marks) plus docToMarkdown, markdownToDoc,
docToPlainText, docToHtml, and extractOutline built on it. Markdown
parsing extends markdown-it's default preset with a token-stream
transform for GFM task lists and table-cell paragraph wrapping.
docToHtml hand-rolls escaping and link-protocol allowlisting with zero
DOM dependencies, so it runs in the API/collab server as well as the
browser.
Node names `wikilink` and `plugin_block` are reserved for later stories.
Closes#24
