Move CONTRIBUTING.md to .github/ and refresh

Author: dereuromark

.github/CONTRIBUTING.md

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+# Contributing
+
+:+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+The issue tracker is not a support forum. Please keep issues to bug reports and
+enhancement proposals. For general CakePHP support, see
+<https://cakephp.org/pages/get-involved#userSupport>.
+
+## How to Contribute
+
+1. Fork the repository on GitHub and create a feature branch from `master`.
+2. Add tests for any new functionality or bugfix (regression test).
+3. Make sure the quality gates pass (see below).
+4. Submit a pull request with a clear description of what changed and why.
+
+## Development Setup
+
+```bash
+composer install
+
+# Run the plugin migrations
+bin/cake migrations migrate -p Queue
+```
+
+## Quality Gates
+
+Please run these before submitting (the same checks CI runs):
+
+```bash
+composer test        # PHPUnit
+composer stan        # PHPStan (run `composer stan-setup` once first)
+composer cs-check    # Coding standards (`composer cs-fix` to auto-fix)
+```
+
+## Coding Standards
+
+This plugin follows the PSR2R coding standards. Please make sure
+`composer cs-check` is green before opening a pull request.
+
+## Updating the Locale POT File
+
+If you change any translatable strings, refresh the plugin's POT file by
+running this from your application directory:
+
+```bash
+bin/cake i18n extract --plugin Queue --extract-core=no --merge=no --overwrite
+```
+
+## Running MySQL Tests
+
+By default the suite uses SQLite (works out of the box). Not all tests run on
+SQLite or other non-MySQL databases yet. To run the full suite including the
+MySQL-only tests:
+
+```bash
+export DB_CLASS=Mysql
+export DB_URL="mysql://root:yourpwd@127.0.0.1/cake_test"
+vendor/bin/phpunit
+```
+
+Make sure the `cake_test` database exists.
+
+## Pull Request Guidelines
+
+- Write clear, descriptive commit messages.
+- Keep each pull request focused on a single feature or bug fix.
+- Update the README/docs when you change user-facing behavior.
+
+## Questions?
+
+Open an issue for discussion if anything is unclear.