Estimate what a build run will cost and what it is capped at

The estimate the Build composer shows next to the prompt: what this prompt text and this project's current size are expected to cost, and the hard cap the run will stop at. The run is metered as it goes and charges step by step, so this is an estimate and the cap is a promise — the run checkpoints and ends with `budget.exhausted` rather than billing past the cap. `affordable: false` is the early warning that precedes the 402.

curl -X GET "https://example.com/billing/estimate?prompt=string"

{  "runEstimate": {    "promptCredits": 0,    "projectCredits": 0,    "total": 0,    "cap": 0  },  "balance": {    "allowance": 0,    "purchased": 0,    "total": 0  },  "affordable": true}