- Require Node.js 20 or later and drop Node 18 from the CI matrix
- Remove the deprecated snake_case validation rules; schemas that still use them throw a TypeError naming the replacement
- Type MailAttachment.content as string | Uint8Array (Buffer still works)
- Bump the version to 11.0.0, finalize the CHANGELOG, and date 10.7.0
- Remove the saga entry that was duplicated into older CHANGELOG releases
- Rename the README migration section to "Migrating to 11.0" and link the migration skill

BREAKING CHANGE: Node.js 20 is required, the snake_case validation rules are removed, and MailAttachment.content no longer names Buffer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>