PR exact-head merge gate

Always freeze the PR base/head, patch hash, and active correctness/security reviewer IDs. Await active reviewers only through automatic completion notifications; never poll, overlap, duplicate, or let stale follow-ups redefine the active set. Empty or progress-only output gets one same-session retry, then replacement; a verdict inspecting other SHAs is invalid and requires a corrected-revision replacement. Require two independent substantive MERGE verdicts on the unchanged patch. Immediately before merging, reconfirm exact head, clean merge/worktree state, green CI/Warden checks, and resolved threads. Merge via gh using an allowed repository method, then verify the immutable merge SHA, parent(s), and tree; for squash merges, the merge tree must equal the reviewed head tree.