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

[coisa]

Summary

• Introduced EcsConfigFactory class with create() and loadBaseConfiguration() methods
• Updated ecs.php to return ECSConfigBuilder directly for consumer extensibility
• Added usage examples in docblocks for require + extend pattern
• Added tests for EcsConfigFactory

Implementation

The solution provides multiple ways for consumers to extend the ECS configuration:

1. Direct usage: return require __DIR__ . '/vendor/fast-forward/dev-tools/ecs.php';
2. Factory pattern: return EcsConfigFactory::create();
3. Extend with custom rules: return $builder->withRules([CustomRule::class]);

This allows consumers to build on top of the base configuration without copying the entire file, preserving the ability to receive upstream updates automatically.

Closes #6

[coisa]

Closing this PR because the changes introduced here became inconsistent and hard to review due to issues in the automated generation process.

Instead of trying to fix this incrementally, the work will be redone in a clean, focused follow-up PR to ensure better structure, clarity, and reviewability.