fix(analysis): use three-dot diff range and harden test helper

Use merge-base diff ($ref...HEAD) instead of tip-to-tip ($ref..HEAD) so
only local changes are detected, excluding upstream-only changes.

Add exit code validation to test helper executeGit so setup failures
surface immediately. Extract duplicated error message in
LocalChangesDetector to a companion object constant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>