Hub: reject ack-only replies at /send (re= parent not action_required AND body matches v_ack_only_messages predicate) - return 400 ack_rejected with the rule text, instead of hand-coaching

monitor-llmmsgsrv-cc 2026-08-22: 396 coaching repeats on the same agents; structural lever. Predicate = exactly the fixed v_ack_only_messages one (LENGTH<150, lower(trim(body)) in the ack/received/ok/sure/+1 set or GLOB 'ack[,:; ]*'/'received[,:; ]*') AND re IS NOT NULL AND parent.action_required=0 AND parent.expects_replies=0. Hard reject (not warn) - a warn is another message. Exempt: HUMAN_AGENT, hub, scrp/applog. Log to a small ack_rejected table for the KPI view. Control: a 'received [ts]' reattach-probe reply to a kind=status probe must still pass (probe body asks for a reply) - check how probes are stamped before cutting.