Add AI Visibility Checker and llms.txt Generator/Validator

[SamHartleyFixes]

Two free tools, one in each of the sections they belong to.

GEO / AI Visibility - AI Visibility Checker. Enter a domain and it reports three things: whether robots.txt allows the AI search crawlers that actually feed citations, whether the homepage returns readable text without JavaScript, and whether an llms.txt is served. The distinction it is built around is that OAI-SearchBot and GPTBot are different crawlers with different consequences, and a lot of sites block both when they meant to block training only.

Validator / Checker - llms.txt generator and validator. The generator reads the site's real homepage and sitemap rather than guessing, and the validator checks a file against the spec, including whether the links inside it resolve.

Both are free with no signup, no ads and no email capture, and results are shareable by URL. The checks are deterministic rather than model-generated, so the same site gives the same answer twice.

The numbers the checker cites come from a census of robots.txt across the Tranco top 5,000, published as open data under CC BY 4.0 with the scanner and the per-domain results at https://github.com/SamHartleyFixes/ai-crawler-census, so anything the tool claims can be checked against the sites it came from.

Happy to split this into two PRs, cut either entry, or shorten the descriptions if they run long for the list.