[qiuethan]

What this fixes

A precreated or replaced automation/sync-main-* branch could pass snapshot validation even when its tree did not represent the merge of its recorded parents. An older staging parent could roll back released files; a snapshot could also discard a production-only hotfix while recording main as merged. This addresses the P1 review on #374.

Snapshot validation now recomputes git merge-tree --write-tree <first-parent> <main> and requires that result to match the snapshot. Conflicts and mismatches stop before opening or merging a sync PR. It also verifies that merging the snapshot into current staging preserves staging's exact tree, catching a production-revert case even when the snapshot matches its own parents. The validation runs again immediately before a merge. Valid snapshots remain reusable after staging advances, preserving concurrent changes and intentional reverts.

Changed-file rationale

File	Why needed
.github/scripts/promotion.cjs	Validate the actual merge represented by every sync snapshot, including reused branches.
.github/scripts/promotion.test.cjs	Reproduce rollback and omitted-hotfix failures with real disposable Git repositories, cover concealed conflicts, and preserve valid retries after staging advances.

Scope stays within .github. There are no configuration, migration or deployment changes. The workflow must receive this fix before promotion to main.

Validation

Fresh CI passed on 286ed790b0699dff7c5163a762dc0cfcb912f5d4.

• The rollback and omitted-hotfix regressions failed against the original implementation. A further production-revert regression demonstrated why current staging must also be checked. All pass after the fix.
• make test-release test-promotion passed, including conflict rejection and valid retry/concurrent-staging cases.
• make check passed with locked dependencies and Python imports pinned to this worktree.
• git diff --check passed. Tests use disposable local Git repositories and fake GitHub calls; no live sync or production promotion was performed.