Allow release promotion to grandfather pre-enforcement commits

[maruson08]

Problem

The new commit-message validator correctly checks pull-request ranges, but the main..v2.1 release-promotion range includes eight published human-authored commits created before enforcement. The release PR would fail even though policy explicitly preserves and grandfathers that history.

Required fix

• Define the last pre-enforcement v2.1 commit as an explicit grandfather boundary
• Exclude ancestors of that boundary when validating a PR range
• Continue validating every later human-authored non-merge commit
• Preserve topology-based merge exclusions, PR-title validation, and branch-policy enforcement
• Add regression coverage for the release-promotion range

No history rewrite, product change, release merge, tag, or deployment is included.

[maruson08]

Resolved by #84, merged into the v2.1 integration branch.