Add AI Agent Guidelines to AGENTS.md

[hyperxpro]

Motivation:

AI coding agents are increasingly used to contribute to AsyncHttpClient. Providing repository-specific guidance helps them follow the project's development practices and produce changes that better align with project expectations.

Modification:

Add an AGENTS.md file defining repository-specific guidance for AI coding agents, including development workflow, testing requirements, coding conventions and contribution expectations.

Result:

AI coding agents produce changes that are more consistent with the project's standards, reducing review overhead and improving contribution quality.