markdown.md 4-backtick rule is over-applied to standalone code blocks (e.g. mermaid)

[bryanchen-d]

Problem

The rule in actions/setup/md/markdown.md:

<markdown-generation><rule>Use 4-backtick fences (not 3) for outer markdown blocks containing nested
code blocks. Use GitHub Flavored Markdown.</rule></markdown-generation>

is being over-applied by the model. It uses 4-backtick fences for all code blocks (e.g. ````mermaid) even when they don't contain any nested triple-backtick fences inside.

This breaks rendering on GitHub — for example, a mermaid diagram wrapped in 4 backticks does not render as a diagram; it shows as raw text.

Observed behavior

In a workflow run (example), the generated comment contained:

````mermaid
graph TD
    A --> B
````

instead of the correct:

```mermaid
graph TD
    A --> B
```

The 4-backtick fence prevents GitHub from recognizing the mermaid block, so the diagram is not rendered.

Additionally, the model sometimes only upgrades the opening fence to 4 backticks while leaving the closing fence at 3, which further breaks formatting.

Expected behavior

The 4-backtick rule should only apply when the code block literally contains triple-backtick fences (```) inside its content. Standalone code blocks like ```mermaid, ```js, ```yaml, etc. should continue to use standard 3-backtick fences.

Suggested fix

Make the rule more precise, for example:

<markdown-generation><rule>Only use 4-backtick fences when the block literally contains triple-backtick
fences inside it. Otherwise use standard 3-backtick fences. Use GitHub Flavored Markdown.</rule></markdown-generation>

This avoids the model conservatively over-applying 4 backticks to blocks that don't need it.