Add extract-line-plot example

[ZhengyaoJiang]

This PR adds the extract-line-plot example which demonstrates optimizing an AI feature that turns chart images into CSV tables using a VLM.

What's included:

• prepare_data.py: Downloads ChartQA and prepares a 100-sample subset of line charts
• optimize.py: Exposes extract_csv(image_path) which returns CSV text plus per-call cost
• eval.py: Evaluation harness that reports accuracy and optional cost metrics
• guide.md: Additional instructions for Weco optimization

Tested:

• Data preparation works
• Baseline evaluation runs successfully (accuracy: 0.613 on 10 samples)

[DhruvSrikanth]

LGTM! Lint/format the code and then feel free to merge.

[aliroberts]

Looks good!