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>

composer.json

@@ -0,0 +1,24 @@
+{
+	"name": "peptiderepo/peptide-repo-core",
+	"description": "Canonical peptide data layer for the peptiderepo.com ecosystem.",
+	"type": "wordpress-plugin",
+	"license": "GPL-2.0-or-later",
+	"require": {
+		"php": ">=8.1"
+	},
+	"require-dev": {
+		"squizlabs/php_codesniffer": "^3.13",
+		"wp-coding-standards/wpcs": "^3.0",
+		"phpcsstandards/phpcsutils": "^1.0",
+		"dealerdirect/phpcodesniffer-composer-installer": "^1.0"
+	},
+	"config": {
+		"allow-plugins": {
+			"dealerdirect/phpcodesniffer-composer-installer": true
+		}
+	},
+	"scripts": {
+		"lint": "phpcs",
+		"lint:fix": "phpcbf"
+	}
+}