chore: track remaining GEO work as issues, remove GEO_PLAN.md (#83)

The prerendering foundation, robots.txt, and basic sitemap shipped in #77.
The remaining GEO work is now tracked as issues #79-#82, so the planning
doc is no longer needed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: schwannden