[ISSUE #4313] : Add AGENTS.md with project structure guidance

[elnafateh]

Describe what this PR does / why we need it

This PR adds a new root-level AGENTS.md file with repository-specific guidance for AI coding assistants working in Spring Cloud Alibaba.

This first change is intentionally narrow and only documents project structure and module layout. It gives contributors and AI assistants a verified starting point for navigating the repository before making code changes.

Does this pull request fix one issue?

Fixes #4313

Describe how you did it

Added AGENTS.md at the repository root and documented:

• the top-level module layout
• the role of major top-level directories
• the structure of spring-cloud-alibaba-starters
• examples of current starter and support modules on 2025.1.x
• guidance on where to start reading depending on the type of change
• basic impact guidance for shared modules such as spring-cloud-alibaba-commons

This PR keeps the scope limited to project structure only and does not add build, JDK, or test workflow guidance yet.

Describe how to verify it

• Open AGENTS.md from the repository root
• Confirm the documented module layout matches the current 2025.1.x branch structure
• Confirm the referenced starter modules exist under spring-cloud-alibaba-starters/
• Confirm no other project files were changed

Special notes for reviews

This is intended as the first incremental contribution for issue #4313.

I kept the PR to a single new file so the initial review can focus on whether the documented repository structure is accurate before expanding AGENTS.md with additional sections in follow-up PRs.