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_

まず、観測された /env の出力を、リンク先の GitHub Copilot CLI エージェント命令に関するドキュメントと比較し、グローバル命令ファイルのデフォルトの検出動作を特定します。完了とは、どのファイルが使用され、重複する命令がどのように処理されるかをドキュメントが正確に説明している状態です。また、/env が重複排除を示しているかどうかも含みます。

索引モデルが issue の本文から書いたものです。