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

Follow-up from July 3 PR #12919 merge audit. At lease: query #12919 merge state. If merged: task event + complete and run any required post-merge hygiene if parent issue context exists. If still open: read at least last 25 Carson DMs and only send a single 3-option ping (merge / hand off / hold) if there is a material PR-state delta or no #12919 nudge in last 24h. Otherwise NO_SEND_DEDUPE. On non-terminal outcome, re-arm +24h.