Spec Kitty

Mission Run: Consolidate git merge-base/diff idiom

Collapse four hand-rolled git merge-base to diff copies onto one shared helper.

Four independent modules each shell out to git merge-base then git diff --name-only with their own error handling and tests. This consolidation routes all four onto a single core/vcs/git.py helper so the changed-files idiom is proven once and cannot drift, extending the milestone no-shadow-paths single-seam discipline to a duplicated VCS primitive.