[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.

Start by reproducing `/model` in Copilot CLI v1.0.59 on a Copilot Free plan and compare the Available tab with the requested Claude access. The issue provides no source file or test entry point; done would mean a clearly defined and approved change that makes at least Claude Sonnet 4.5 available under the stated Free-plan limits.

索引モデルが issue の本文から書いたものです。