Clarify startup message "No copilot-instructions.md found" to specify it means the repo-scoped file

Describe the feature or problem you'd like to solve

On session start, the CLI prints "No copilot-instructions.md found." This wording is ambiguous.

Proposed solution

On session start, the CLI prints "No copilot-instructions.md found." This wording is ambiguous — it reads as though NO custom instructions were loaded at all, when in fact it only means the repository-scoped file (.github/copilot-instructions.md in the git root/cwd) is absent.

A user-scope file at $HOME/.copilot/copilot-instructions.md (and other supported sources like AGENTS.md, CLAUDE.md, $HOME/.copilot/instructions/**) may still be fully loaded and active. The current message caused me to believe none of my instructions were in effect, when my user-scope instructions actually were loaded.

Requested change / proposed solution: reword the message to name the specific missing file/scope, e.g. "No repository-scoped .github/copilot-instructions.md found" — and ideally note that user-scope or other instruction sources may still be active.

Benefits to GitHub Copilot CLI and its users:

• Removes a misleading signal that makes users think their instructions aren't loaded.
• Reduces confusion and wasted time spent debugging a non-issue.
• Sets accurate expectations about which instruction scope is missing vs. active.
• Encourages correct use of layered instruction sources (repo-scoped vs. user-scoped).

Example prompts or workflows

1. User has a populated $HOME/.copilot/copilot-instructions.md with personal workflows.
2. User starts a session in a repo that has no .github/copilot-instructions.md.
3. CLI prints "No copilot-instructions.md found."
4. User reasonably concludes NONE of their instructions loaded — but the user-scope file is active.
5. Running /env confirms the user-scope instructions ARE loaded, contradicting the startup message.

Suggested improved message:
"No repository-scoped .github/copilot-instructions.md found. (User-scope and other instruction sources, if present, are still active — see /env or /instructions.)"

Additional context

从发出 "No copilot-instructions.md found." 的 CLI 会话启动路径开始，跟踪如何检查仓库范围的指令。更新措辞，以指出缺少的仓库范围文件，同时保留用户范围和其他指令源的行为；在启动期间以及使用 /env 或 /instructions 时验证结果。