Add requirements_completed field to summary-extract output, mapping from SUMMARY frontmatter requirements-completed key. Enables /gsd:audit-milestone cross-check to receive data from SUMMARY source. Re-applied from #631 against refactored codebase (commands.cjs + split tests). Co-authored-by: Colin Johnson <Solvely-Colin@users.noreply.github.com> Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>