Move 13 shipped openspec change docs under openspec/changes/archived/.
Add docs/features/git-diff-panel, docs/plans/post-review-backlog, and
docs/research/cross-app-contract-ssot.md (the research behind the
@boocode/contracts SSOT work). Update BOOCHAT.md, BOOCODER.md, and
boocode_roadmap.md.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>