Literal CRAN 2.0 release (pre-pkgdown updates)

Keep AGENTS.md out of the published pkgdown site

pkgdown::build_home() auto-publishes every top-level *.md file except
a small hardcoded exemption list (README.md, LICENSE.md, NEWS.md, a
few CI files) -- there's no _pkgdown.yml option to add to it, so
AGENTS.md was getting built into docs/AGENTS.md and docs/AGENTS.html
and listed in sitemap.xml. It needs to stay at the repo root (that's
the convention agents look for), so pkgdown/remove-internal-pages.R
removes the generated page and its sitemap entry after each
pkgdown::build_site() run instead. Documented as a build step at the
top of AGENTS.md, and updated the tutorials section to describe the
current standalone-page architecture (tutorials/ + build-webr-
tutorials.R) as the primary guidance, with the pkgdown-article history
condensed into a "why not to go back to that" section.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>