Use two-tip diffs for long-lived environment promotions

Reusable promotion-audit method: for long-lived environment branches with independently promoted/cherry-picked work, assess the release delta with a two-tip tree diff (git diff target..release), not only a three-dot merge-base diff. A three-dot diff can falsely classify changes already present on the target as new. In negotiation PR #1012, the three-dot view made three leaderboard migrations appear new; the two-tip view showed only one migration was actually entering main. Pair this with an explicit release-tree identity check after conflict resolution. Observed 2026-09-04T18:20Z from local Git commit/tree comparisons for e3-solutions/negotiation, origin/main, origin/staging, and PR #1012.