fix(ci): restructure CI for PHP 8.1+ and proper PHPCS setup

- Drop PHP 8.0 from matrix — DTOs use readonly properties (8.1+)
- Add composer.json with PHPCS/WPCS as dev dependencies
- Add phpcs.xml.dist for WordPress coding standard config
- Use dealerdirect/phpcodesniffer-composer-installer for auto path setup
- Separate lint jobs: PHP syntax (8.1/8.2/8.3) + PHPCS + file-size
- Update Requires PHP header to 8.1
- Update ARCHITECTURE.md file tree

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

peptide-repo-core.php

@@ -8,7 +8,7 @@
  * Author URI:  https://peptiderepo.com
  * License:     GPL-2.0-or-later
  * Text Domain: peptide-repo-core
- * Requires PHP: 8.0
+ * Requires PHP: 8.1
  *
  * @see ARCHITECTURE.md — Full data flow and file tree.
  * @see CONVENTIONS.md  — Naming patterns and extension guides.