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>

ARCHITECTURE.md

@@ -48,10 +48,12 @@ peptide-repo-core/
 ├── ARCHITECTURE.md                    # This file
 ├── CONVENTIONS.md                     # Naming patterns, extension guides
 ├── CHANGELOG.md                       # Semantic versioning changelog
+├── composer.json                      # Dev dependencies (PHPCS, WPCS) and lint scripts
+├── phpcs.xml.dist                     # PHPCS ruleset configuration
 │
 ├── .github/
 │   └── workflows/
-│       └── ci.yml                     # PHP lint (8.0-8.2), PHPCS, 300-line check
+│       └── ci.yml                     # PHP lint (8.1-8.3), PHPCS WordPress, 300-line check
 │
 ├── assets/
 │   └── css/