Per the v0.18.0 email + webhook hygiene proposal
(~/git/ohm-infra/RFC-APP-EMAIL-HYGIENE-PROPOSAL.md §1), this is the
shared envelope builder every send path will call in Slice 2. No
call-site changes yet — Slice 2 migrates email_otc / email_invite /
email._deliver / email._send_bundle to use it.
The helper centralizes the deliverability-critical headers (Date,
Message-ID, Auto-Submitted) and exposes per-kind unsubscribe
semantics (none for OTC, mailto: for invites, full one-click for
watcher notifications) as explicit kwargs rather than buried in
each call site.
15 new unit tests covering: always-present headers, Auto-Submitted
toggle, the three List-Unsubscribe shapes, plain-only vs
multipart/alternative body. Full suite: 267 passed (was 252).
Ben Stull