Show local machine hostname in Copilot CLI interface

## Summary
Please include the local machine hostname in the Copilot CLI interface context panel.

## Motivation
When running multiple shells/machines (local + remote, containers, SSH sessions), it is easy to lose track of where a command will run. Showing hostname directly in the interface would reduce mistakes.

## Requested behavior
Add a field in the environment context section, e.g.:

- `Hostname: my-machine`

## Notes
- Prefer the actual runtime hostname from the executing environment
- Keep existing context fields unchanged
- This should work across Linux/macOS/Windows

首先定位 Copilot CLI 环境上下文部分，以及提供其现有上下文字段的代码。跟踪在 Linux、macOS 和 Windows 上如何获取运行时环境详细信息，然后验证 hostname 是否显示在上下文面板中，同时现有字段保持不变。

由索引模型根据 Issue 内容生成。