feat: users can symlink model binaries

[0xSage]

Problem

• Users don't want duplicated model binaries
• Users may want various model.json configuration files for a single model

Success Criteria A clear and concise description of what you want to happen.

Questions:

1. Should repurpose the model.json > source_url property for this? Or something else? I'm out of the loop on this.
2. How to ensure backwards compatibility if so?

source_url: RELATIVE_PATH_TO_MODEL_BINARY
# versus
source_url: REMOTE_URL

Not in scope yet

• Files/rag

Additional context Add any other context or screenshots about the feature request here.

• Related: #1010