Add a Priority field to DraftProjection populated from the EML header pair
X-Cli-Priority (CLI/OAPI primary) → X-Priority (RFC fallback for IMAP-回灌
historical drafts), with case-insensitive lookup via the existing
headerValue helper and a local mapping table aligned with the backend
gopkg/mail_priority.PriorityValueToType vocabulary. When neither header is
present (the symmetric read of --set-priority normal=remove_header) the
projection emits "unknown" so agents have a stable read-side surface.
Append one notes entry to buildDraftEditPatchTemplate documenting the
--set-priority flag and the X-Cli-Priority translation contract.
The write-side (--set-priority flag, parsePriority helper, translation
branch in mail_draft_edit.go, EML header target) is unchanged — already
shipped on master.
sprint: S4