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

src/Clock.php

@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace App;
@@ -8,8 +9,10 @@ use Psr\Clock\ClockInterface;
 use Symfony\Component\DependencyInjection\Attribute\AsAlias;
 
 #[AsAlias(ClockInterface::class)]
-final readonly class Clock implements ClockInterface {
-    public function now(): DateTimeImmutable {
+final readonly class Clock implements ClockInterface
+{
+    public function now(): DateTimeImmutable
+    {
         return new DateTimeImmutable();
     }
 }