Detached PR worktrees need an activation path that verifies the intended remote branch

Observation (2026-09-05, source: COR-4057 PR #798 worktree + linear-progress-sync activation): activating an existing Linear/PR record with its exact branch still leaves the repository write guard unable to verify a detached worktree, so even a git merge --ff-only origin/<PR-branch> is blocked. For repair tasks that must remain detached, the guard needs a safe verification path based on the activated branch plus the checked-out commit ancestry, rather than requiring git branch --show-current.