Independent reviewer retry and resumption

Always keep the exact review tree frozen and the merge gate closed until every required independent reviewer returns substantive file-level evidence or an explicit blocker. Empty output gets one same-reviewer retry; after a second empty result, replace only that reviewer. After a server restart, resume existing reviewers with their context instead of duplicating work. Never sleep, poll, request status, or review the same topics while an agent is active, because parallel duplication breaks independence and wastes work.