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.

The issue does not identify files, tests, or entry points. Start by locating the existing model configuration and model-selection flow in Copilot CLI, then determine how an additional provider such as DeepSeek or Kimi would be represented. Done means users can choose a lower-cost or open-weight option without replacing the current approach.

Scritto dal modello di indicizzazione a partire dal testo della issue.