feat: add layout and content hook points

Wire the hook.html system into the base layout and single templates so site
projects can inject or filter markup without overriding templates. Adds the
positional hooks head-end, body-start, header-before, header-after,
main-before, main-after, footer-before, footer-after and body-end in
baseof.html (head-end via head-additions.html), plus content-before and
content-after around the article content in single.html and page/single.html.

Each hook is a no-op until a site provides the matching
layouts/_partials/hooks/<name>.html partial.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>