Global instructions.md AGENTS.md CLAUDE.md documentation clarification

### Describe the feature or problem you'd like to solve

Provide clear documentation about default behavior of Global instruction md files utilization

### Proposed solution

Provide clear documentation about default behavior of Global instruction md files utilization

This will avoid confusion for end users

There is currently some documentation around this (https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/add-custom-instructions#agent-instructions) although seems to be slightly unclear/inaccurate in comparison to observed behavior of CLI

example (running on Windows os, WSL Ubuntu):
running `/env`:

however, I only have AGENTS.md in ~/.agents and CLAUDE.md in ~/.claude

_additionally for the above, a nice enhancement with respect to release notes of version 1.0.26 - 2026-04-14 "Avoid sending duplicate custom instruction files (e.g. copilot-instructions.md and CLAUDE.md with identical content) to reduce wasted tokens per turn", it would be nice here on the `/env` to demark as "(duplicate)" label suffix or something equivalent there to alleviate user concerns that these may bloat context/instructions sent and that they are properly being de-duplicated by copilot harness_

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先，将观察到的 /env 输出与链接的 GitHub Copilot CLI 代理指令文档进行比较，并确定全局指令文件的默认发现行为。完成的标准是文档准确说明使用哪些文件以及如何处理重复指令，包括 /env 是否表明进行了去重。

由索引模型根据 Issue 内容生成。