Cost Confirmation

## Goal
People know how much their benchmark test is going to cost them in credits before running it.

## Requirements
Display to the user how much the benchmark will cost and prompt the user to confirm before proceeding.

The issue names no files, tests, or entry points. Start by locating the benchmark execution flow and the credit-cost calculation, then trace where the user is prompted before a run proceeds. Done means the estimated cost is shown and the benchmark waits for explicit confirmation.

Written by the indexing model from the issue text.