Add AGENTS.md coding guide for AI agents

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

## Description
I would like to contribute an `AGENTS.md` file that documents Apache Fluss
coding conventions, patterns, and standards specifically for AI coding
agents.

## Motivation
AI coding assistants (like Claude Code, GitHub Copilot, etc.) can provide
better suggestions when they have project-specific context. This guide
helps AI agents understand:
- Critical rules enforced by Checkstyle
- API design patterns (builders, factories, interface segregation)
- Testing conventions (JUnit 5, AssertJ)
- Module boundaries and dependencies
- Configuration patterns

## Approach
The documentation extracts patterns from the existing codebase:
- Critical rules from `tools/maven/checkstyle.xml`
- Code patterns from production code
- Testing patterns from test utilities
- All examples are real code from Apache Fluss

### Willingness to contribute

- [x] I'm willing to submit a PR!

Start with tools/maven/checkstyle.xml, then inspect representative production code and test utilities for the conventions and patterns named in the issue. The work is done when a new AGENTS.md documents Checkstyle rules, API and configuration patterns, testing conventions, and module boundaries with examples drawn from Apache Fluss.

Written by the indexing model from the issue text.