feat: add proxy cost summary to cost report

[simonrosenberg]

Summary

• aggregate test_result.proxy_cost values when generating cost_report.jsonl
• add a new top-level proxy_cost_summary field to the report JSON
• leave the existing summary.total_cost behavior unchanged

Output shape

The generated cost_report.jsonl now includes:

"proxy_cost_summary": {
  "total_proxy_cost": ...,
  "only_main_output_proxy_cost": ...,
  "sum_critic_files_proxy_cost": ...
}

Scope

This PR only changes report generation. It does not add proxy_cost to per-instance outputs by itself.

Testing

• pytest -q tests/test_report_costs.py
• python3 -m py_compile benchmarks/utils/report_costs.py

Closes #603

[all-hands-bot]

🟢 Good taste - Clean implementation that follows the existing cost-tracking pattern, maintains backward compatibility, and includes tests. The parallel structure (separate proxy_cost_summary vs mixing into summary) is the right call—it keeps the distinction between accumulated_cost and proxy_cost clear.

KEY INSIGHT: Pragmatic extension of existing reporting logic without breaking the "no double-count" semantics. Ship it.