Adds ImportModelFile tool (import_model_file command) that copies an
on-disk model file (FBX/OBJ/GLB/glTF/zip) under Assets/ and runs it
through the shared ModelImportPipeline. Returns {asset_path, asset_guid}.
Adds ImportModelFileHandlerTests covering missing-source, unsupported
extension, and a real OBJ import.
Claude-Session: https://claude.ai/code/session_01NoHk4f7N1vUFs7gu817ihm