A Markdown file starting with YAML front matter (agent SKILL.md files,
static site pages) showed it in the preview as a thematic break and a run
of text. The preview now shows every top-level field in a key / value
table above the body; nested lists and mappings keep their YAML layout.
Blocks after it keep their source lines for scroll sync. Text between two
thematic breaks that isn't a mapping is rendered as before.
Fixes#1239