Artifact: Scoped vs. Open-Ended Prompt -- the Cost Comparison

The same bug fix, prompted two ways, compared by token cost -- exercises co-18.

Prompt style	Task	Turns	Total tokens	Total cost
Open-ended: "the retry logic is flaky, fix it"	same bug fix	6	48,200	$0.72
Scoped: names carrier_adapter/retry.py, the failing assertion, and the fix constraint	same bug fix	2	11,400	$0.17

The open-ended prompt spent its first three turns locating the flaky test, reading the retry module, and forming a hypothesis before making any change -- work the scoped prompt's author had already done and simply stated up front.