feat: Add links to other llms-full files to /docs/llms-full.txt

[royanger]

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What does this solve? What changed?

• Prepends a descriptive header to the generated llms-full.txt, including a brief description and links to companion llms-full.txt files (top-level index, articles, blog, changelog).
• Exports LLMS_FULL_HEADER from scripts/lib/llms.ts and updates the build test to use it.
• Adds a missing description to docs/guides/development/deployment/staging-alternatives.mdx.

Adds the following to the top of /docs/llms-full.txt

## Companion files

- [All sections index](https://clerk.com/llms-full.txt): Top-level index linking to every llms-full.txt file on clerk.com
- [Articles](https://clerk.com/articles/llms-full.txt): Full content of all Clerk articles
- [Blog](https://clerk.com/blog/llms-full.txt): Full content of all Clerk blog posts
- [Changelog](https://clerk.com/changelog/llms-full.txt): Full content of all Clerk changelog entries

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Deadline

• https://github.com/clerk/clerk/pull/2507 must be merged first.

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Project	Deployment	Actions	Updated (UTC)
clerk-docs	Ready	Preview	May 13, 2026 4:17pm

[manovotny]

Pushed a small test fix directly in 9dcc12d.

• Inlined the expected llms-full.txt header text in scripts/build-docs.test.ts instead of interpolating ${LLMS_FULL_HEADER}. The previous form would still pass if the header URLs or wording were silently changed in scripts/lib/llms.ts; the inlined version now fails loudly on any unintended edit.

[manovotny]

Approving, but I did add the do not merge label because https://github.com/clerk/clerk/pull/2507 needs to be merged first, otherwise the companion links will fail/404.