docs: add AGENTS.md for AI contributors

[wustwyh]

Summary

Adds an AGENTS.md file to help AI coding agents quickly understand the multi-language layout, per-language setup pointers, and contribution requirements of the ADK Samples repository.

The repository already has README.md and CONTRIBUTING.md, but lacks a concise reference aimed at automated agents. This file fills that gap without changing any sample code.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation only
• Refactor / internal cleanup

Test Plan

• Verified the Markdown renders correctly.
• Confirmed all referenced folders exist at the repository root.
• No code changes, so no functional tests required.

Related Issues

None.

Notes for Reviewers

This is a documentation-only addition.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.