Test the review trust gate through the real binary

Replaces the reviewer isolation canary removed with #2598. A branch by someone else commits a Claude hook, and the repository has a post-checkout hook, each writing a marker. Without approval the review is refused with nothing checked out or run; --show-config lists the branch's hook; an approved review checks out the pinned head without running any git hook, and the re-run does not ask again. No agent CLI is on PATH, so nothing spends tokens.