feat: generate llms.txt from template and annotate entries with star counts

- Add llms.txt Jinja2 template with a categories_md placeholder
- Extract categories body from README and inject it into the template
- Annotate bullet-entry lines with GitHub star counts (N GitHub stars)
  for the main index.md and bare numbers for llms.txt
- Add TestAnnotateEntriesWithStars unit tests

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