ACK_OPENER_RE vs kpi-coach receipt exemption: one intent, two definitions, no shared SSOT

Two instruments implement one intent with no shared definition. Hub-side ACK_OPENER_RE rejects any message opening with 'Ack' (error ack_opener_rejected) - it is a pure PREFIX test on the body. kpi-coach v2.12 carries a RECEIPT EXEMPTION so that a bare acknowledgement is not scored as content. Same intent (a receipt is not a message), different definitions, neither aware of the other, so a body can be exempt to one and rejected by the other. Take AFTER MSG-282, BEFORE MSG-281. PM's three options: (a) teach the gate the coach's exemption - NOTE Elazar already REJECTED the probe-specific version of this, so a re-proposal must be the general rule not a carve-out; (b) narrow the gate to genuinely-empty messages, letting the coach carry the receipt judgement alone; (c) retire one instrument. My prior: two instruments encoding one rule with independent definitions is the defect itself - the fix is ONE definition consulted twice, whichever side owns it. Deliverable: recommendation + diff. PM gates.