Add example: sentiment analysis on CSV with cost tracking #89

Description

Create a complete, self-contained example script showing a real-world sentiment analysis workflow with cost tracking enabled. This helps new users see Ondine in action for a common use case.

What to do

1. Create examples/sentiment_analysis_with_costs.py
2. Include: loading a sample CSV, running sentiment classification, printing cost summary
3. Use QuickPipeline for simplicity
4. Add inline comments explaining each step
5. Include a small sample CSV (5-10 rows) or generate one in the script

Acceptance criteria

• Script runs end-to-end (with a valid API key)
• Cost summary is printed at the end
• Clear inline comments for newcomers