Custom frontmatter headers: write your own Key: value fields under the
subject, and carry a sender's fields into a reply.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Trims the body before sealing instead of appending a newline, and hands an
accept token to a contact once rather than attaching it to every message.
Both made a post from this client unpublishable by a service that republishes
what it receives.