chore: simplify CLAUDE.md for agent use

[agiannelli-auth0]

Description

Trims CLAUDE.md down to behavioral rules only and moves the full Mintlify component reference into a new WRITING_GUIDE.md.

The original CLAUDE.md had grown to ~900 lines — mostly a writing style guide and component reference that's useful for humans but burns tokens on every Claude turn without changing agent behavior. This separates the two concerns:

• CLAUDE.md is now ~25 lines of imperative rules: layout, build commands, key conventions, and guardrails. Every line changes what Claude does.
• WRITING_GUIDE.md captures the full Mintlify component reference, admonition usage guide, placeholder conventions, and writing principles — everything a human (or triggered skill) needs, without paying the ambient context cost.

References

Hackathon!

Testing

Checklist

• I've read and followed CONTRIBUTING.md.
• I've tested the site build for this change locally.
• I've made appropriate docs updates for any code or config changes.
• I've coordinated with the Product Docs and/or Docs Management team about non-trivial changes.

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