Adds the /grok SEO landing page (and its auto-generated route/sitemap
entries), the /agents directory card, and the Grok line in the ACP
catalog docs, following the Grok Build provider added to the in-app
catalog.
Every agent landing page now mentions both mobile and desktop, leads with
"open source", and uses a search-friendly hero title. Renames Pi to "Pi Agent"
and disambiguates other common-word names in titles.
