llms.txt SECTIONS list is hand-maintained in parallel with nav #178

SECTIONS in hooks/emit_markdown.py is a manual list of pages that mirrors nav in mkdocs.yml. It's complete today, but a new page added to nav won't appear in llms.txt unless someone also edits the hook.

Fix: derive the section/page list from config.nav so the two can't drift (or, at minimum, add a comment in mkdocs.yml pointing at the hook). Follow-up to #176.