docs: three-tier integration guide + announcement draft; house style sweep

INTEGRATION.md gives other teams three adoption paths: zero-code invoice
links, the drop-in widget (git install or CDN import), and headless
webhook confirmation, plus the honest privacy table and merchant sweep
guidance. Also a builders-group announcement draft and a repo-wide
typographic pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>