[Feature Request] Move `.github/copilot-instructions.md` to `AGENTS.md`

[njzjz]

Summary

Move .github/copilot-instructions.md to AGENTS.md since the latter is common used by different agents.

Detailed Description

As above.

Further Information, Files, and Links

https://github.blog/changelog/2025-08-28-copilot-coding-agent-now-supports-agents-md-custom-instructions/