refactor: split migration cli command

[brianvarskonst]

Summary

Splits the WP-CLI migration command into context, executor, and reporter collaborators, adds rollback failure coverage, and adopts shared QA tooling.

Checklist

• The change is focused and scoped to one concern.
• Tests or verification notes are included for behavior changes.
• Documentation was added or updated where needed.
• Public APIs, configuration keys, and contracts are described clearly.
• Backward compatibility impact is noted.

Type of Change

• Bug fix
• Feature
• Documentation
• Refactoring
• Maintenance
• Breaking change

Verification

• git diff --check origin/main...HEAD
• Not run locally: PHP/composer are not available in this shell; package CI should run the PHP QA commands.

Notes

Branch is rebased on the upstream changelog and PHPUnit cache cleanup.

[brianvarskonst]

Reviewed the package-level diff and CI result. The CLI split, rollback coverage, WordPress SQL adapter fixes, and changelog update are separated into conventional commits, and GitHub Actions is green. No blocking findings.