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/ClockTest.php

@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace App\Tests\Unit;
@@ -6,8 +7,10 @@ namespace App\Tests\Unit;
 use App\Clock;
 use PHPUnit\Framework\TestCase;
 
-final class ClockTest extends TestCase {
-    public function testNowReturnsDateTimeImmutable(): void {
+final class ClockTest extends TestCase
+{
+    public function testNowReturnsDateTimeImmutable(): void
+    {
         $clock = new Clock();
         $before = new \DateTimeImmutable();
         $now = $clock->now();