feat: support multiple reference images with model-aware API mapping

Replace singular reference_image field with reference_images list to
support an arbitrary number of reference images. Map them to the correct
BFL API parameter names based on model family:
- flux-2-*: input_image, input_image_2, ..., input_image_8
- flux-kontext-*: input_image, input_image_2, ..., input_image_4
- flux 1.x: image_prompt (single)

BREAKING CHANGE: reference_image config field renamed to reference_images (list).

tests/test_graph.py

@@ -66,7 +66,7 @@ class TestBuildGraph:
     ) -> None:
         _ = (project_dir / "ref.png").write_bytes(b"\x89PNG")
         config = write_config(
-            {"targets": {"out.png": {"prompt": "x", "reference_image": "ref.png"}}}
+            {"targets": {"out.png": {"prompt": "x", "reference_images": ["ref.png"]}}}
         )
         graph = build_graph(config, project_dir)
         assert graph.has_edge("ref.png", "out.png")