chore: add php-cs-fixer with PSR-12 config and CI check

- Add friendsofphp/php-cs-fixer to require-dev
- Create .php-cs-fixer.dist.php configured for @PSR12 ruleset
- Add php-cs-fixer dry-run step to CI pipeline
- Auto-fix existing PSR-12 violations
- Document code style tooling in readme.md

.php-cs-fixer.dist.php

@@ -0,0 +1,18 @@
+<?php
+
+$finder = (new PhpCsFixer\Finder())
+    ->in(__DIR__)
+    ->exclude('var')
+    ->exclude('vendor')
+    ->notPath([
+        'config/bundles.php',
+        'config/reference.php',
+    ])
+;
+
+return (new PhpCsFixer\Config())
+    ->setRules([
+        '@PSR12' => true,
+    ])
+    ->setFinder($finder)
+;