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

tests/Unit/Trait/CookieNameTraitTest.php

@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace App\Tests\Unit\Trait;
@@ -6,18 +7,22 @@ namespace App\Tests\Unit\Trait;
 use App\Trait\CookieNameTrait;
 use PHPUnit\Framework\TestCase;
 
-final class CookieNameTraitTest extends TestCase {
+final class CookieNameTraitTest extends TestCase
+{
     use CookieNameTrait;
 
-    public function testCookieName(): void {
+    public function testCookieName(): void
+    {
         self::assertSame('__Host-Http-Preauth', $this->cookieName());
     }
 
-    public function testAuthCookieName(): void {
+    public function testAuthCookieName(): void
+    {
         self::assertSame('__Http-Domain-Preauth', $this->authCookieName());
     }
 
-    public function testHeaderName(): void {
+    public function testHeaderName(): void
+    {
         self::assertSame('X-Preauth', $this->headerName());
     }
 }