Add Pull Request reviews to Push MD

[artpi]

Summary

• store preview branches as a private push_md_pull_request CPT with native lifecycle statuses
• add WordPress Notes for general and inline diff comments
• add comment-driven, extensible PR review states
• add editable PR descriptions and commit history with commit message bodies
• add a GitHub-like review UI with previews, diffs, lifecycle groups, and merge actions
• migrate legacy branch preview options and remove PR data on uninstall

REST and permissions

• use the native post and comment REST controllers
• expose calculated changed files, line diffs, preview URLs, and commit history as read-only fields
• allow admins to update only the description of active PRs
• keep lifecycle, protected metadata, merged/closed PRs, and state-changing Notes guarded

Testing

• vendor/bin/phpcs plugins/push-md
• vendor/bin/phpunit plugins/push-md/Tests/
• full WordPress E2E: 31 tests, 698 assertions
• node --check plugins/push-md/admin-shell.js
• git diff --check