[docs] Add AGENTS.md - AI agent coding guide

[vaibhavk1992]

This documentation extracts and documents coding conventions, patterns, and standards from the existing Apache Fluss codebase to assist AI coding agents. All rules and examples are derived from actual source code, Checkstyle configuration, and build files.

• 11 comprehensive sections covering critical rules, API patterns, testing, dependencies, configuration, and build/CI
• 100+ concrete code examples with DO/DON'T comparisons
• Direct file references to canonical examples in the codebase
• Fully compliant with Apache generative AI guidelines

Also updated .gitignore to exclude CLAUDE.md (personal development notes)

Purpose

Linked issue: #2921

Brief change log

Tests

API and Format

Documentation

[qzyu999]

Hi @vaibhavk1992, thanks for the initial PR! Just sharing some quick observations. I took a look and it looks quite solid with lots of useful information for coding agents. For comparison I examined Airflow's AGENTS.md (https://github.com/apache/airflow/blob/main/AGENTS.md).

I noticed that in Airflow's AGENTS.md they have a section that tells the AI how to properly run git commands in addition to identifying as AI-generated: https://github.com/apache/airflow/blob/main/AGENTS.md#creating-pull-requests

There's also a section of explicit boundaries for the agent that may have some overlap for Fluss: https://github.com/apache/airflow/blob/main/AGENTS.md#boundaries

I noticed that the AGENTS.md files here and in Airflow both focus primarily on code contribution rather than deployment. I've read that AGENTS.md could serve a dual purpose: guiding contributors, and helping developers use AI to quickly set up and deploy a new repository from day one.

However, I do believe the saying is that AGENTS.md is intended to be relatively brief (<500 lines?), and this document has already exceeded that. I am not sure how this could be best balanced.

Thanks again for your work on this.

Edit: A workaround for the lengthier AGENTS.md is to provide a breadcrumbs route, pointing the agent towards further documentation in other subfolders for additional information (e.g., /docs/contributing_code.md). Additionally, (I think it was mentioned elsewhere) it's possible to have AGENTS.md nested in submodules.