feat: add OpenAI as provider for text and image generation

- Add openai_text.py: text generation via OpenAI chat completions API
  (gpt-4o, gpt-4o-mini, gpt-4.1, gpt-4.1-mini, gpt-4.1-nano, o3-mini)
- Add openai_image.py: image generation via OpenAI images API
  (gpt-image-1 with reference image support, dall-e-3, dall-e-2)
- Refactor builder provider dispatch from TargetType to model-name index
  to support multiple providers per target type
- Fix circular import between config.py and providers/__init__.py
  using TYPE_CHECKING guard
- Fix stale default model assertions in tests
- Add openai>=1.0.0 dependency

tests/test_config.py

@@ -22,8 +22,8 @@ class TestLoadConfig:
 
         assert "out.txt" in config.targets
         assert config.targets["out.txt"].prompt == "hello"
-        assert config.defaults.text_model == "mistral-large-latest"
-        assert config.defaults.image_model == "flux-pro-1.1"
+        assert config.defaults.text_model == "pixtral-large-latest"
+        assert config.defaults.image_model == "flux-2-pro"
 
     def test_full_config_with_all_fields(self, project_dir: Path) -> None:
         raw = {