[feature] Make ecs.php extensible instead of fully replaceable

[coisa]

Summary

• Added ECSConfig class in FastForward\DevTools\Config namespace with static configure() method
• The method returns ECSConfigBuilder with the default ECS configuration
• Updated ecs.php to use the new class
• Added PHPDoc documentation explaining how to extend the configuration in consumer projects

Changes

• Created src/Config/ECSConfig.php with static configure() method
• Updated ecs.php to use ECSConfig::configure()

Usage Example

In a consumer project, create ecs.php that extends the default configuration:

use FastForward\DevTools\Config\ECSConfig;
use PhpCsFixer\Fixer\Phpdoc\PhpdocAlignFixer;

$config = ECSConfig::configure();
$config->withRules([CustomRule::class]);
$config->withConfiguredRule(PhpdocAlignFixer::class, ['align' => 'right']);

return $config;

This allows consumers to:

• Build upon the default configuration without copying the entire file
• Automatically receive upstream updates to the base configuration
• Only override what they need

Testing

All tests pass (88 tests, 243 assertions).

Closes #6