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

Follow-up from 2026-07-07 PR #12919 merge audit. At lease: (1) query PR #12919 state. If MERGED/CLOSED, task event + complete and run any required post-merge hygiene if parent issue context exists. (2) If OPEN: refresh checks/reviews/merge state. (3) Read last 25 Carson DMs. Do NOT send another DM unless there is a material PR-state delta (merged/closed/reopened, failing CI, new review feedback, reviewDecision/mergeStateStatus changed) or Carson directly asks. A clock-only change is NO_SEND_DEDUPE. (4) If still open/no material delta, complete NO_SEND_DEDUPE and re-arm +24h if appropriate. Agents do not merge TextureHQ/mono. Targeted decision ping was Slack ts 1783260614.694909 on 2026-07-05T14:10Z; no repeat ping unless >72h since that ping AND material send gate passes.