[FEAT] new --save-dataset-cache arg

[cdreetz]

Description

added new save-dataset-cache arg to eval.py centralizing information across verifiers usage or at least providing the option to i think is generally good

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

new args:

vf-eval my-env -sc
vf-eval my-env --save-dataset-cache
defaults to using ~/.cache/verifiers/

christian@Christians-MacBook-Pro:~/.cache/verifiers$ ls
evals
christian@Christians-MacBook-Pro:~/.cache/verifiers$ ls evals
index.json		mcp-env--gpt-4.1-mini
christian@Christians-MacBook-Pro:~/.cache/verifiers$ cat evals/index.json
{
  "mcp-env--gpt-4.1-mini/82149437": {
    "env": "mcp-env",
    "model": "gpt-4.1-mini",
    "timestamp": "2025-09-20T01:15:21.271069",
    "path": "/Users/christian/.cache/verifiers/evals/mcp-env--gpt-4.1-mini/82149437",
    "avg_reward": 0.0,
    "num_examples": 1,
    "rollouts_per_example": 1
  }
} 
christian@Christians-MacBook-Pro:~/.cache/verifiers$ ls evals/mcp-env--gpt-4.1-mini
82149437	bf98fd91
christian@Christians-MacBook-Pro:~/.cache/verifiers$ ls evals/mcp-env--gpt-4.1-mini/82149437
metadata.json	results.jsonl
christian@Christians-MacBook-Pro:~/.cache/verifiers$