System requirement doc for different Platform of sandboxing feature

### Describe the bug

Hi, team , we're trying to integration copilot sandboxing feature to IntelliJ copilot plugin. However, seems sandbox feature is not very stable, and we cannot make it work on some platforms.

On Windows:

From docs, it should work for version 26300.8553, however, one machine with this version works fine and other one show errors:
`Experimental_CreateProcessInSandbox returned E_NOTIMPL. The following velocity keys are not enabled: 61389575, 61155944.`

Two machine has the same version as below:
```
Edition Windows 11 Enterprise Insider Preview
Version 26H2
Installed on ‎2026/‎7/‎6
Evaluation expires on ‎2026/‎8/‎12 2:09
OS build 26300.8758
Experience Windows Feature Experience Pack 1000.26100.402.0
```

On Linux:

It always show command failed after enable sandbox, maybe there is a version requirement?

On Mac:
When a sandboxed command is launched on macOS Seatbelt with a process working directory that is not allowed by the sandbox policy, the child shell can print noisy getcwd warnings during startup.

The command itself may still succeed, especially when it uses an absolute path that is allowed by the policy. The issue is confusing stderr noise, not a command failure .

I think this is related MXC and we already create an issue here: https://github.com/microsoft/mxc/issues/611

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

首先检查 Windows、Linux 和 macOS 上报告的 sandbox 行为，以及链接的 MXC issue #611。记录平台要求、版本限制和已知的 macOS 警告行为，并明确说明每个平台支持和不支持的情况。

由索引模型根据 Issue 内容生成。