Add The Rookery to the llms.txt directory

Site: The Rookery (https://rookery.online)
llms.txt: https://rookery.online/llms.txt (conforms to the spec: H1 title, blockquote summary, H2 sections, absolute markdown links - validated)
Category: Websites / Products
We also ship an llms.txt-aware agent registry at /citizens.json.

Review the repository's existing llms.txt directory entries and follow their format and category placement. Add The Rookery with its site URL, validated llms.txt URL, and Websites / Products category; done means the entry is consistent with neighboring entries and both links are preserved.

Written by the indexing model from the issue text.