fix: frontmatter rendering as removal

[paoloricciuti]

In sveltejs/ai-tools#188 I've included the subagent definition which is a markdown codeblock with the frontmatter.

The current rendering strategy detect that as a removal because of

---
[content]
---

and you can see how bad it looks

This fixes while still leaving the ability to do diffs inside markdown blocks (it just exclude the actual frontmatter) in md and yml codeblocks.

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Project	Deployment	Actions	Updated (UTC)
svelte-dev	Ready	Preview	Apr 10, 2026 4:09pm

[jycouet]

• ci is failing because since [email protected] it's pulling [email protected] and that brings this type issue. I pushed an override to see. fix: make @typescript-eslint/types an optional peer dependency esrap#108 would help
• now: (but I have to go right now)

apps/svelte.dev check: Error: Type '() => ReturnType<import("svelte").Snippet>' is not assignable to type 'Snippet<[]>'.

• Other than that, the real change you want looks good ^^