Support lower-cost/open-weight model options to improve affordability

**Token-based usage is a good direction, but the current cost curve can make usage feel expensive very quickly.** For many users, that becomes a barrier to continued use and slower growth.

## Why this matters

- **Affordability:** heavy usage can become too expensive too fast
- **Choice:** users should be able to pick a model that fits their budget and workload
- **Retention:** lower-cost options can make Copilot easier to keep using long term

## Proposal

Consider supporting **lower-cost or open-weight models** such as **DeepSeek** or **Kimi** as additional options.

That could make Copilot:
- more affordable
- more flexible
- less punishing for users who burn through tokens quickly
- better aligned with different usage levels

**This is not a request to replace the current approach** — just to add more affordable model options so users can choose what fits their budget and workload.

此 issue 未指出檔案、測試或進入點。先在 Copilot CLI 中找出現有的模型設定與模型選擇流程，然後確定如何表示 DeepSeek 或 Kimi 之類的額外 provider。完成的標準是使用者可以選擇成本較低或 open-weight 的選項，而不必取代目前的方法。

由索引模型根據 Issue 內容生成。