Add AGENTS.md

[andygrove]

What is the problem the feature request solves?

Most of the DataFusion projects now have an AGENTS.md:

• https://github.com/apache/datafusion/blob/main/AGENTS.md
• https://github.com/apache/datafusion-python/blob/main/AGENTS.md
• https://github.com/apache/datafusion-sqlparser-rs/blob/main/AGENTS.md

I think it would be beneficial to add one to Comet as well.

Describe the potential solution

No response

Additional context

No response