Fix website tool options silently ignored under the book key

[cderv]

HTML books are websites, and the documented contract is that website tools go under the book key in a book project rather than under website, as stated in https://quarto.org/docs/books/book-output.html#website-tools. Five options that the book schema accepts did nothing there: plausible-analytics (#14879), llms-txt (#14276), announcement (#10114), plus back-to-top-navigation and image-alt, which had no issue filed.

Root cause

bookProjectConfig in src/project/types/book/book-config.ts implements that contract by copying a hand-maintained list of website keys from book into the internal website config. Keys added to base-website in the schema after that list was written were never added to it, so they passed validation and were then dropped with no warning. google-analytics is on the list, which is why it works in a book while plausible-analytics did not.

Nothing beyond the copy was missing. Each of the five works as soon as the value reaches the website config, so the hooks that consume them are already reachable from a book render. That is worth noting for #14276, whose description attributes the llms-txt failure to formatExtras not being inherited by books: src/project/types/book/book.ts does call websiteProjectType.formatExtras, and the llms.txt index generates correctly once the key is copied.

drafts and draft-mode are absent from the list too and are left alone here. They filter which files a project renders rather than adding an HTML tool, and #9233 tracks them as a feature request.

Note on the top-level website key

Setting one of these five under a top-level website key in a book project happened to work before, because a key absent from the copy list was left untouched. They are now read from book like every other website tool in a book, so that form no longer applies.

A companion quarto-web PR moves the book-versus-website key note so it is visible from the analytics section of the website tools page, where it was previously 150 lines above.

The second commit is unrelated test hygiene: two llms-txt website tests render with output-dir: . and did not ignore the search.json and site_libs/ they produce, so any run of the website suite left untracked files behind.

Closes #10114, closes #14276, closes #14879

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[cderv]

Docs companion: quarto-dev/quarto-web#2186. The book-vs-website key note already existed, it just sat under Social Metadata while Analytics is well below it, so a reader landing on the Analytics anchor never saw it. Moved to a page-level intro and added a short pointer inside Analytics.