docs: add AGENTS.md with build, test, and Spark diff guidance (#4083)

Author: andygrove

AGENTS.md

@@ -0,0 +1,35 @@
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Agent Guidelines for Apache DataFusion Comet
+
+Read the [Contributor Guide](docs/source/contributor-guide/index.md) before making changes.
+Relevant entry points:
+
+- [Development Guide](docs/source/contributor-guide/development.md): build, test, and common
+  pitfalls (including why `-pl` must not be used and the JVM/native build order).
+- [Spark SQL Tests](docs/source/contributor-guide/spark-sql-tests.md): 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](docs/source/contributor-guide/adding_a_new_expression.md) /
+  [Adding a New Operator](docs/source/contributor-guide/adding_a_new_operator.md).
+- [Debugging Guide](docs/source/contributor-guide/debugging.md).
+
+When opening a pull request, use the [PR template](.github/pull_request_template.md) and fill
+in every section.