ops(guardrails): enforce merge-commit headline on auto-merge

[c-wri]

Summary
The repo needs an enforced path for merge-commit headlines so GitHub auto-merge does not land multi-checkpoint PRs with the default Merge pull request ... subject on main.

Problem
The repo standard requires multi-checkpoint PRs to land with a merge commit whose subject is <pr title> (#<pr number>). PR #84 merged through GitHub auto-merge, but the resulting merge commit on main used GitHub's default headline instead. That leaves durable mainline history out of policy even when the PR title and body were correct.

Evidence

• PR feat(normalization): land downstream target products #84 merged successfully through the protected-branch PR path.
• The resulting merge commit is 296485e1bef13218674bdef5fee9bb5042a70c92 on main.
• Its subject is Merge pull request #84 from CrownOpsEng/feat/economic-facts-reconciliation-checkpoint instead of the required PR-title-based merge subject.

Desired Outcome
The repo should have a supported, enforced merge path that keeps multi-checkpoint PR merges on the required headline format without relying on manual post-merge repair or rewriting merged main history.

Acceptance Criteria

• The repo documents and enforces the supported merge path for multi-checkpoint PRs so the merge commit subject matches <pr title> (#<pr number>).
• The enforcement path works with the repo's protected-branch PR-only flow and does not require rewriting merged main commits.
• The repo's delivery or CI guardrails catch any future merge path that would fall back to GitHub's default Merge pull request ... subject.