feat: ensure image output format matches file extension

- Add hokusai/image.py with Pillow-based format detection and conversion
- Pass output_format to OpenAI gpt-image models and BFL API
- Convert mismatched images after provider writes (fallback for all providers)
- Handle RGBA-to-RGB flattening for JPEG targets
- Add Pillow dependency

pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
     "mistralai>=1.0.0",
     "networkx>=3.6.1",
     "openai>=2.21.0",
+    "pillow>=11.0.0",
     "pydantic>=2.12.5",
     "pyyaml>=6.0",
     "typer>=0.23.1",