feat: mark spam moves mail to the account's spam folder

Mark spam was a tag-only action that added the spam tag and removed inbox,
so a message marked as spam stayed in the inbox on disk. It now MOVES the
file into the account's configured spam folder, exactly mirroring Delete:
the account-relative spam key is the destination, the move records
moved-from: with the origin, and unread and inbox are stripped in the same
confirmed write so one undo returns the folder and the tags together.

NotmuchWorker::moveMessages already handled a folder generically and
applyTags already overwrote an older moved-from: tag, so the worker needed
no change; the tests pin that behaviour for the spam destination.

Five existing tests used spam as a worker-free, tag-only stand-in for the
old Delete. Since spam is now a move too, they are retargeted to flag, the
remaining selection-scoped tag-only action.

0 files changed, 0 insertions, 0 deletions