feat: add cost estimates

A cost estimation feature that does one or more of the following:

1. Post-operation cost summary – After a code generation task is completed, display an estimate of the tokens used and the corresponding API cost.

1. Pre-operation cost preview – Based on the prompt size or estimated tokens, show a rough cost estimate before the assistant starts generating code.

1. Live cost tracker – For longer or interactive sessions, a running counter showing token usage and estimated cost in real-time would be incredibly useful.