[Repo Assist] fix: handle EmbedParagraphs in Markdown.ToMd serialiser

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Summary

Markdown.ToMd (the Markdown-to-Markdown serialiser in MarkdownUtils.formatParagraph) did not handle the EmbedParagraphs AST node. It fell through to a catch-all | _ branch that:

1. Printed a debug message to stdout: // can't yet format <EmbedParagraphs …> to markdown
2. Yielded an empty string, silently dropping the paragraph's content

Both the HTML formatter (HtmlFormatting.fs line 169) and the LaTeX formatter (LatexFormatting.fs line 124) already handle EmbedParagraphs by calling cmd.Render() and recursing. This PR brings MarkdownUtils.formatParagraph into consistency with them.

Root cause

The | EmbedParagraphs(cmd, _) case was simply missing from formatParagraph in MarkdownUtils.fs.

Fix

Add:

| EmbedParagraphs(cmd, _) -> yield! cmd.Render() |> Seq.collect (formatParagraph ctx)
```

Once all `MarkdownParagraph` DU cases are explicitly handled, the old `| _` catch-all becomes dead code (the compiler now warns FS0026 on it), so it is removed. The exhaustiveness checker will catch any future `MarkdownParagraph` case that lacks a `formatParagraph` clause.

## Changes

- `src/FSharp.Formatting.Markdown/MarkdownUtils.fs` — add `EmbedParagraphs` case; remove dead catch-all
- `tests/FSharp.Markdown.Tests/Markdown.fs` — new test verifying `ToMd` delegates to `Render()`
- `RELEASE_NOTES.md` — entry under `[Unreleased]`

## Test Status

```
Build succeeded.
    0 Warning(s)
    0 Error(s)

Passed!  - Failed: 0, Passed: 282, Skipped: 0, Total: 282  (FSharp.Markdown.Tests)
Passed!  - Failed: 0, Passed: 143, Skipped: 0, Total: 143  (FSharp.Literate.Tests)
Passed!  - Failed: 0, Passed:  88, Skipped: 4, Total:  92  (FSharp.ApiDocs.Tests)
Passed!  - Failed: 0, Passed:  30, Skipped: 2, Total:  32  (FSharp.CodeFormat.Tests)
Passed!  - Failed: 0, Passed:  12, Skipped: 0, Total:  12  (fsdocs-tool.Tests)

All tests pass; new test ToMd serialises EmbedParagraphs by delegating to Render() confirms the fix.

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