Squash-merge of PR #22: adopt shared reusable CI workflow, convert PHPUnit to stubs, update branch-protection contexts.
QA-approved at 05d8094a1d. All required CI checks green.
Agent-Session: 2026-06-17-prcore-ci-deploy
1. NAMESPACE is a PHP reserved keyword — renamed to REST_NAMESPACE
in the REST controller. This caused a parse error on activation.
2. Autoloader can't resolve PR_Core class — the class name equals the
autoloader prefix exactly, so stripping "PR_Core_" yields an empty
suffix and the wrong filename. Added explicit require_once in
bootstrap, same pattern as PRAutoBlogger.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>