PR #12919 merge audit re-arm: check merge state; single 3-option DM only if material delta and dedupe gate passes

PR #12919 merge-audit re-arm. At lease: (1) query PR state — if MERGED/CLOSED, task event + complete. (2) If OPEN: compute staleness = hours since last commit push or last review activity. (3) Dedupe basis (CORRECTED 2026-07-05): a standup body mentioning #12919 as one line in a status summary does NOT count as a decision-ping. Only a targeted DM/thread reply naming #12919 with a decision-frame (3-option, merge/close/pause) counts. Any Kai targeted-decision-ping to Carson referencing #12919 in last 24h → NO_SEND_DEDUPE. Otherwise if staleness >72h AND PR still mergeable+approved → send single in-thread 3-option DM in existing Slack DM channel. Ping-once-then-back-off per Core Rule #11 + CHECKLIST-CONTACT.md. (4) Self-re-arm at +24h regardless of outcome. Capture dedupe_basis + staleness_hours + decision_ping_last_24h boolean in result.