The scope drift step (Step 1.5) used `git diff origin/<base> --stat`
(two-dot), which shows the full tree difference between the branch tip
and the base ref. On rebased branches this includes commits already on
the base branch, producing false-positive "scope drift" findings for
changes the author did not introduce.
Switch to `git diff origin/<base>...HEAD --stat` (three-dot / merge-base
diff), which shows only changes introduced on the feature branch. This
matches what /ship already uses for its line-count stat.
