Fix main merge release classifier

[erotonin]

Summary

• classify main merge commits by diffing HEAD^1..HEAD
• apply the same merge-aware changed-file detection before opening promotion PRs

Validation

• pre-commit run --all-files
• git diff --name-only origin/main^1 origin/main shows non-GitOps files for the prior merge

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud