bwi hub posts omit x-llmmsg-client, tripping the hub client-outdated backstop

wi_hub_post() in ~/.local/bin/bwi (line 43) POSTs /send with content-type + Authorization only. The hub's backstopUnrenderable (hub.mjs:6690-6694) fires on exactly one condition - absence of the x-llmmsg-client header - so every bwi close/announce DMs the acting agent a spurious [client-outdated] warning listing outcome, recipients_reading, origin_aro, rate-limited to 1/sender/hour. Measured 2026-10-04: coder-terra-cc got one from 'bwi close TERRA-221' (msg 138738 -> warning 138739). A headered vs headerless pair through https://llmmsg-hub.pensanta.com reproduced both directions - headered tag mutyjchwjbjo: no warning; headerless tag mutyjdb8yja5: warning 138747 - and both responses carried all 7 keys, so nothing is actually being discarded. Fix: add -H 'x-llmmsg-client: bwi/<version>' in wi_hub_post. A hub-side exemption was considered and rejected: it would blind the detector to a whole caller class, and a declared client string is the existing contract. Not a hub change.