The @tiptap/markdown extension discovers serializers via the
renderMarkdown extension field, not addStorage(). The previous
addStorage approach was silently ignored, causing mentions to serialize
as shortcode format [@ id="..." label="..."] instead of markdown links.
Now properly overrides renderMarkdown, parseMarkdown, and
markdownTokenizer to serialize mentions as [@Label](mention://type/id)
which the Markdown renderer can handle as clickable links.