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

Bắt đầu từ đường dẫn khởi động phiên CLI phát ra "No copilot-instructions.md found." và theo dõi cách các chỉ dẫn theo phạm vi repository được kiểm tra. Cập nhật nội dung để xác định tệp theo phạm vi repository bị thiếu, đồng thời giữ nguyên hành vi của phạm vi người dùng và các nguồn chỉ dẫn khác; xác minh kết quả trong quá trình khởi động và bằng /env hoặc /instructions.