docs: move copilot-instructions.md to AGENTS.md (#4982)

This PR moves `.github/copilot-instructions.md` to `AGENTS.md` in the
repository root to follow the standard convention used by different AI
agents.

## Background

According to the [GitHub blog
post](https://github.blog/changelog/2025-08-28-copilot-coding-agent-now-supports-agents-md-custom-instructions/),
`AGENTS.md` is the commonly used filename for agent instructions that
can be recognized by various AI coding agents, not just GitHub Copilot.

## Changes

- Moved `.github/copilot-instructions.md` to `AGENTS.md` in the
repository root
- Preserved all existing content exactly (191 lines of detailed
instructions)
- Removed the old file location

## Benefits

- Makes agent instructions discoverable by multiple AI coding agents
- Follows the established convention in the AI development community
- Maintains all the valuable build, test, and development guidance for
the DeePMD-kit repository

No functional changes were made - this is purely a file relocation to
improve discoverability and standardization.

Fixes #4981.

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: njzjz <9496702+njzjz@users.noreply.github.com>

Author: Copilot