Add LLM cost tracking and budgeting

[aviralgarg05]

Summary

Add cost tracking and estimation for LLM-based evaluations.

Tasks

• Track token usage in trace events.
• Calculate estimated cost based on model pricing.
• Add --cost-limit flag to abort if budget exceeded.
• Include cost summary in reports.

Acceptance criteria

• Token counts captured in trace.
• Cost estimates shown in results.
• Evaluation stops gracefully at cost limit.