chore: add llms.txt for the repository and spock.sh

[softmarshmallow]

Goal

Add an llms.txt entry point for Spock and publish the same content at spock.sh/llms.txt.

The file should give AI agents a concise, reliable map of Spock's authoritative documentation without creating a second documentation source of truth.

Implementation notes

• Prefer one canonical tracked source, likely /llms.txt at the repository root.
• Have the Astro/Vercel website serve that canonical content at /llms.txt.
• Avoid maintaining separate hand-written copies in the repository root and www/public/.
• Link to the most useful stable resources, including:
  • project and framework overview
  • getting started documentation
  • current language specification
  • GraphQL dialect specification
  • design records
  • governance and contribution guidance
  • GitHub source
• Keep the content compact and useful as an index; do not duplicate the full documentation corpus.
• Document how the published file is generated or copied as part of the website build.

Acceptance criteria

• A canonical llms.txt source is tracked in the repository.
• https://spock.sh/llms.txt returns HTTP 200 as plain text.
• The published content matches the canonical repository source.
• The file points to stable, authoritative Spock documentation URLs.
• The website build or checks prevent the published copy from silently drifting.