[Copilot Free] Only Claude Haiku 4.5 available — request access to Claude Sonnet/Opus models

## Summary

On the Copilot Free plan, the only available Claude model is **Claude Haiku 4.5**.
Claude Sonnet and Opus models are locked behind the Upgrade tab and require a paid plan.

## Current behavior

When running `/model` in Copilot CLI (Agency v2026.6.2.8), the "Available" tab shows only:
- Auto
- GPT-5 mini (default)
- Claude Haiku 4.5

## Expected behavior

At least **Claude Sonnet 4.5** should be available on the Free plan, even with a lower
rate limit or higher credit cost per request. Haiku is fast but lacks the reasoning depth
needed for complex coding tasks, making it hard for free-tier users to evaluate Claude's
full potential within Copilot.

## Environment

- OS: Windows 11 (ARM64)
- Agency version: 2026.6.2.8
- Copilot CLI version: v1.0.59
- Plan: Copilot Free

## Additional context

GPT-5 mini is provided as the default free model. It would be great to have a comparable
mid-tier Claude option (Sonnet) available as well, so users can make an informed choice
between model families before deciding to upgrade.

首先，在 Copilot Free 计划上于 Copilot CLI v1.0.59 中复现`/model`，并将 Available 选项卡与所请求的 Claude 访问权限进行比较。该 issue 未提供源文件或测试入口；完成的标准是有一个定义清晰且获得批准的更改，使至少 Claude Sonnet 4.5 在所述 Free 计划限制内可用。

由索引模型根据 Issue 内容生成。