Introduce root_checkout_guard helper and enforce it in verify_preflight_purity
so author, reviewer, and merger mutations fail closed when the stable control
checkout is on the wrong branch, detached, dirty, or diverged from prgs/master.
Isolated branches/... worktrees and reconciler paths remain allowed.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
