Generate LLM-friendly complete text at `/llms.txt`

[jaggederest]

This is the modern version of SEO, where you get your product included in the LLM training corpus and become standard for vibe coding I guess.

[codexbounty260804]

Implemented this in #106 using Sourcey 3.6.5. The generated root llms.txt covers 62 documentation pages, and I verified every generated target returns HTTP 200. Feedback welcome.