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_

Start by reviewing the reported Windows, Linux, and macOS sandbox behavior and the linked MXC issue #611. Document the platform requirements, version constraints, and known macOS warning behavior, with clear supported and unsupported cases for each platform.

Written by the indexing model from the issue text.