Agent Guidelines for Apache DataFusion Comet

Read the Contributor Guide before making changes. Relevant entry points:

• Development Guide: build, test, and common pitfalls (including why -pl must not be used and the JVM/native build order).
• Spark SQL Tests: the only supported way to modify files under dev/diffs/. Never hand-edit a diff file. Clone Spark, apply the existing diff, modify the Spark source, then regenerate the diff as documented there.
• Adding a New Expression / Adding a New Operator.
• Debugging Guide.

When opening a pull request, use the PR template and fill in every section.