Upgrade to PHPUnit 10, add PHPCS with WPCS compliance, add phpcs CI job

- Upgrade PHPUnit 9.6 → 10, update phpunit.xml.dist schema
- Add PHPCS 3.13 with WordPress-Extra + PHPCompatibilityWP standards
- PHPCBF auto-fix + manual fixes for full WPCS compliance
- Add phpcs job to release workflow (parallel with lint)
- Pin composer platform to PHP 8.3 to prevent incompatible dep locks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -18,7 +18,8 @@ This project is 100% AI-generated ("vibe-coded") using Claude.AI.
 - **Styling:** Custom CSS
 - **Dependencies:** Composer
 - **i18n:** WordPress i18n (.pot/.po/.mo), text domain: `wc-composable-product`
-- **Testing:** PHPUnit 9.6 + Brain Monkey 2.7 + Mockery 1.6
+- **Testing:** PHPUnit 10 + Brain Monkey 2.7 + Mockery 1.6
+- **Linting:** PHPCS 3.13 with WordPress-Extra + PHPCompatibilityWP
 - **CI/CD:** Gitea Actions (`.gitea/workflows/release.yml`)
 
 ## Project Structure
@@ -55,6 +56,7 @@ wc-composable-product/
 │   └── product-selector.twig  # Frontend selection interface
 ├── vendor/                    # Composer dependencies (gitignored, included in releases)
 ├── composer.json
+├── phpcs.xml.dist             # PHPCS configuration (WordPress-Extra + PHPCompatibilityWP)
 ├── phpunit.xml.dist           # PHPUnit configuration
 └── wc-composable-product.php  # Main plugin file
 ```
@@ -140,11 +142,15 @@ Compile .po to .mo: `for po in languages/*.po; do msgfmt -o "${po%.po}.mo" "$po"
 
 WordPress requires compiled .mo files — .po files alone are insufficient.
 
-## Testing
+## Testing & Linting
 
 Run unit tests: `vendor/bin/phpunit --testdox`
 
-Tests use **Brain Monkey** to mock WordPress/WooCommerce functions without a full WP installation. WooCommerce classes (`WC_Product`, `WC_Settings_Page`, etc.) are provided as minimal stubs in `tests/stubs/` so PHP can parse `extends` declarations. The release workflow runs tests before building — a failing test blocks the release.
+Run coding standards check: `vendor/bin/phpcs`
+
+Auto-fix coding standard violations: `vendor/bin/phpcbf`
+
+Tests use **Brain Monkey** to mock WordPress/WooCommerce functions without a full WP installation. WooCommerce classes (`WC_Product`, `WC_Settings_Page`, etc.) are provided as minimal stubs in `tests/stubs/` so PHP can parse `extends` declarations. PHPCS uses the **WordPress-Extra** standard plus **PHPCompatibilityWP** for PHP version checks. The release workflow runs lint, phpcs, and tests before building — any failure blocks the release.
 
 ## Release Workflow