Publish a minimal llms.txt for the Lat website

Summary

• Add a minimal repository-root llms.txt introducing Lat, explaining installation and lat init, and directing agents to browse the homepage for current documentation. No deep documentation links while content is being updated.
• Copy the file unchanged into .lat-build/server/public/ from both website build commands; Vercel's existing packaging carries it into static output at /llms.txt.
• Keep this specific to Lat's website: no changes to the general-purpose exporter or user-generated sites.
• Update documentation and add a regression test for the copy step and minimal contents.

Validation

• pnpm buildall
• pnpm test — 322 tests
• node dist/src/cli/index.js check
• Byte-for-byte comparison of root llms.txt with the copied website artifact