feat: document which API model (and price) each Kimi Code plan model id corresponds to #4781

We run Kimi Code CLI next to other coding agents and record each turn's token usage from wire.jsonl, so a team can compare the cost of the same work across models and providers. For accounts signed in with /login (the coding plan), we can't turn those tokens into an API-equivalent price. Tested on 2.3.1, macOS arm64.

What we see:

• wire.jsonl and the model catalog carry the plan's model ids (kimi-for-coding, kimi-for-coding-highspeed, k3, k3-256k, k3.1-pro, k3.1-pro-highspeed), not the API platform's ids (kimi-k3.1-pro, kimi-k3, kimi-k2.7-code, …).
• The API pricing page (platform.kimi.ai/docs/pricing/chat) lists prices only by API id.
• The display names suggest a mapping (for example k3.1-pro → kimi-k3.1-pro), but we found it documented nowhere. kimi-for-coding (K2.8 Preview) has no API price at all. We'd rather not guess, because a wrong mapping gives silently wrong numbers.

Ask:

1. Document the mapping: for each coding-plan model id, the API model it corresponds to (or "no API equivalent"), on the Kimi Code docs or the pricing page.
2. Or expose it in the model catalog: an optional field on each plan model entry, for example "api_model": "kimi-k3.1-pro", next to the id that wire.jsonl already records.

Either one would let tools show an API-equivalent cost for plan usage, label it as such, and keep it correct when models change. A smaller, related request: publishing the 5-hour window and the monthly pool per plan in the same units would let tools turn "% used" into an expected amount of work.

Related: #2106 (session cost in the footer, closed).

Happy to test a canary build.