Fix llms.txt title generation and add AI-surface validation

[portdeveloper]

What changed

• Removes inline code from three recipe H1 headings so Vocs generates complete llms.txt titles.
• Adds pnpm check:ai-surface to validate generated AI-facing files after pnpm build.
• Checks generated llms.txt, llms-full.txt, robots.txt, sitemap.xml, and SKILL.md exist and catch malformed AI index entries.

Why

The public llms.txt index was emitting truncated titles for headings with inline code, which makes AI retrieval and page selection less reliable.

Fixes #164

Merge order

Independent. This PR can be merged in any order relative to #165 and #166.

Validation

• pnpm build
• pnpm check:ai-surface

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