feat(pricing): refresh bundled prices and consider an explicit user update API

Why this matters

Chatlas packages a snapshot of ellmer pricing data for cost estimates. The current snapshot is older than ellmer's source: chatlas has 1,286 entries dated 2026-08-10, while ellmer has 1,306 entries dated 2026-08-17. This omits pricing for newer models and tiers, including Claude Mythos, Gemini 3.7, GPT-5.6 Bedrock large-context tiers, and newer Groq/Mistral models.

Ellmer added an explicit models_update_prices() cache-refresh API in tidyverse/ellmer#968 (57e3d72d) after deciding against automatic background refreshes.

Scope

1. Refresh the bundled chatlas/data/prices.json from ellmer as a maintenance update.
2. Evaluate whether chatlas should expose a user-invoked pricing refresh API backed by a local cache, rather than requiring a package release for new model pricing.
3. If adding the API, define its network behavior, cache location, schema/version validation, failure behavior, and whether cached data replaces or merges with bundled data. Keep it explicit: no automatic network request during normal imports or chats.
4. Add tests that do not require live network access.

Starting points

• chatlas/_tokens.py: loads the packaged pricing snapshot at import time and performs lookups.
• Makefile target update-pricing: downloads the source artifact from ellmer.
• .github/workflows/check-update-types.yml: keeps generated pricing current in CI.
• Ellmer reference: git -C ../ellmer show 57e3d72d.

Non-goals

• Do not change cost calculation semantics or make pricing lookup network-dependent.
• Do not silently alter prices during import, tests, or a normal chat request.

Open design question

The bundled-data refresh is independently actionable. The runtime API is optional and should be split into a follow-up if its design adds more than a small, well-tested cache layer.

Start with chatlas/_tokens.py and the Makefile update-pricing target, then inspect .github/workflows/check-update-types.yml and ellmer commit 57e3d72d. The bundled-data portion is done when chatlas/data/prices.json matches the current ellmer source; any refresh API should have offline tests and must not make normal imports or chats access the network.