PR #12919 merge audit re-arm: post-decision-ping check merge state; no repeat DM unless material delta

Post-decision-ping audit for #12919. Trigger 2026-07-07T14:10Z. Read last 25 Carson DMs; if targeted decision ping already sent within last 24h with no material PR-state delta (mergeStateStatus change, new CODEOWNER comment, or Carson reply), NO_SEND_DEDUPE. Only SEND if material delta AND >72h since last ping. Re-arm +24h on complete. Ping-once-then-back-off active since 2026-07-05T14:10Z targeted DM (slack_ts=1783260614.694909). Do not double-ping.