[Repo Assist] fix: Markdown.ToMd preserves YAML frontmatter

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Summary

Markdown.ToMd was silently dropping YAML front-matter blocks when serialising a parsed MarkdownDocument back to Markdown text. Any document opened with a ---...--- front-matter block would lose it on round-trip.

Root Cause

In MarkdownUtils.fs, the formatParagraph function had:

| YamlFrontmatter _ -> ()

This matched the YamlFrontmatter AST node and emitted nothing. For HTML and LaTeX output formats this is correct (front-matter is metadata, not content), but for the Markdown serialiser (ToMd) it means the front-matter is permanently lost.

Fix

Emit the --- delimiters and the front-matter lines in formatParagraph:

| YamlFrontmatter(lines, _) ->
    yield "---"
    for line in lines do
        yield line
    yield "---"
    yield ""

Tests

Two new [<Test>] cases added to FSharp.Markdown.Tests:

• ToMd preserves YAML frontmatter — round-trips a document with populated front-matter
• ToMd preserves empty YAML frontmatter — round-trips a document with empty ---\n--- block

Test Status

• ✅ FSharp.Markdown.Tests: 283 passed, 0 failed (2 new tests added)
• ✅ FSharp.Literate.Tests: 143 passed, 0 failed
• i️ FSharp.ApiDocs.Tests: 80 pre-existing failures on main (unrelated to this change, verified by running on unmodified main)

Generated by 🌈 Repo Assist, see workflow run. Learn more.

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@97143ac59cb3a13ef2a77581f929f06719c7402a