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>

.github/workflows/ci.yml

@@ -6,15 +6,17 @@ on:
   pull_request:
     branches: [ main ]
 
-jobs:
-  lint:
-    name: PHP Lint + PHPCS
-    runs-on: ubuntu-latest
+permissions:
+  contents: read
 
+jobs:
+  # ── PHP Syntax Check ─────────────────────────────────────────────
+  lint-php:
+    name: PHP Lint (${{ matrix.php }})
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: [ '8.0', '8.1', '8.2' ]
-
+        php: [ '8.1', '8.2', '8.3' ]
     steps:
       - uses: actions/checkout@v4
 
@@ -22,37 +24,30 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
-          tools: composer, cs2pr
-          extensions: mbstring
 
-      - name: PHP syntax check
+      - name: Syntax check all PHP files
         run: find . -name "*.php" -not -path "./vendor/*" | xargs -n 1 php -l
 
-      - name: Install PHPCS + WordPress standards
-        run: |
-          composer global require --with-all-dependencies \
-            "squizlabs/php_codesniffer=^3.13" \
-            "wp-coding-standards/wpcs=^3.0" \
-            "phpcsstandards/phpcsutils=^1.0"
-          phpcs --config-set installed_paths $(composer global config home)/vendor/wp-coding-standards/wpcs
+  # ── WordPress Coding Standards (PHP CodeSniffer) ──────────────────
+  lint-phpcs:
+    name: WordPress Coding Standards
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.1'
+
+      - name: Install Composer dependencies
+        run: composer install --no-interaction --prefer-dist --no-progress
 
       - name: Run PHPCS
-        run: |
-          phpcs --standard=WordPress --extensions=php \
-            --ignore=vendor/*,node_modules/*,tests/* \
-            --report=checkstyle \
-            --warning-severity=0 \
-            . | cs2pr || true
-
-      - name: Run PHPCS (strict — fail on errors)
-        run: |
-          phpcs --standard=WordPress --extensions=php \
-            --ignore=vendor/*,node_modules/*,tests/* \
-            --error-severity=1 --warning-severity=0 \
-            --report=summary \
-            .
+        run: composer run lint
         continue-on-error: true
 
+  # ── 300-line file check ──────────────────────────────────────────
   file-size:
     name: 300-line file check
     runs-on: ubuntu-latest