fix: add merge_group trigger to CI workflow

[CybotTM]

Summary

• Add merge_group: event trigger to CI workflow so merge queue checks run properly
• Without this trigger, PRs get stuck in the merge queue waiting for checks that never start

Test plan

• CI checks pass on this PR
• Merge queue properly triggers checks after merge

[gemini-code-assist]

Note

Gemini is unable to generate a summary for this pull request due to the file types involved not being currently supported.

[Copilot]

Pull request overview

Adds merge_group to the CI workflow triggers so GitHub merge queue runs CI checks as expected, preventing queued PRs from waiting on checks that never start.

Changes:

• Add merge_group: event trigger to .github/workflows/ci.yml

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[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[github-actions]

Thanks for your first pull request! We are excited to have you contribute.
A maintainer will review your PR soon. Please ensure all CI checks pass.

[github-actions]

Automated approval for solo maintainer project

This PR has passed all automated quality gates:

• ✅ Static analysis (PHPStan)
• ✅ Code style (PHP-CS-Fixer)
• ✅ Unit & functional tests
• ✅ Security scanning
• ✅ Dependency review

See SECURITY_CONTROLS.md for compensating controls documentation.