Resolve GGUF artifact links

[hannibal-lee]

What

• Resolve ranked GGUF recommendations to their actual downloadable artifact repo/file.
• Add artifact metadata to ranking results and JSON output.
• Update Rich/Markdown output so model links can point to the real GGUF repo while preserving the recommended base model ID.

Why

Fixes #137.

Some recommendations are scored as an official base model plus a GGUF quantization, but the actual GGUF files may live in a third-party repo. The output previously linked only to the base model repo, so
users could click a Q3_K_M recommendation and land on a repo that does not contain that GGUF artifact.

Testing

• Tests pass (pytest)
• New tests added (if applicable)
• Tested on real hardware (if hardware-related)

Manual verification:

• Ran git diff --check.
• Ran python -m compileall src/whichllm tests/test_artifacts.py tests/test_markdown_output.py tests/test_cli.py.
• Could not run full pytest locally because uv run pytest failed while building pyarrow==23.0.1, and the local Python environments do not have pytest installed.

Notes

This keeps ranking based on the recommended/base model, but exposes the resolved GGUF artifact repo and filename for output/download surfaces.