The Provider API does not expose pricing yet. Overlay a static MODEL_COSTS
map on top of the dynamically fetched model list so pi can display per-model
costs. Models not in the table fall back to zero cost. When the Provider API
adds a cost field, the static table can be removed.
Document the incremental reasoning-delta streaming, live display pricing,
and dynamic model fetch in CHANGELOG.md. Add a Pricing section to
README.md explaining the MODEL_COSTS overlay and how to update prices.
Fei Yan