GH-50373: [Dev][Developer Tools] Add root AGENTS.md file

AGENTS.md

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+<!---
+  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.
+-->
+
+# AGENTS.md
+
+This file provides guidance for AI coding agents working on Apache Arrow.
+
+Apache Arrow is a multi-language toolbox for accelerated data interchange
+and in-memory processing.
+
+## Repository Layout
+
+| Directory | Description |
+|-----------|-------------|
+| `cpp/` | Core C++ library (libarrow, Parquet, Flight, Gandiva, etc.) |
+| `python/` | PyArrow (Python bindings for libarrow) |
+| `r/` | R package |
+| `c_glib/` | C/GLib bindings |
+| `ruby/` | Ruby bindings |
+| `matlab/` | MATLAB implementation |
+| `docs/` | Sphinx documentation source |
+| `ci/` | CI configuration (Docker, scripts) |
+| `dev/` | Developer tools and release scripts |
+| `format/` | Arrow columnar format specification |
+| `testing/` | Shared test data (integration testing) |
+
+Several implementations have moved to their own repositories:
+
+- **Java**: [apache/arrow-java](https://github.com/apache/arrow-java)
+- **Go**: [apache/arrow-go](https://github.com/apache/arrow-go)
+- **Rust**: [apache/arrow-rs](https://github.com/apache/arrow-rs)
+- **C#**: [apache/arrow-csharp](https://github.com/apache/arrow-csharp)
+- **JavaScript**: [apache/arrow-js](https://github.com/apache/arrow-js)
+- **Swift**: [apache/arrow-swift](https://github.com/apache/arrow-swift)
+- **Julia**: [apache/arrow-julia](https://github.com/apache/arrow-julia)
+
+## Issue Conventions
+
+- Prefix the issue title with the component name in brackets:
+  `[C++][Python] Short description of the problem`
+- Do NOT include a `GH-` prefix in issue titles (that is only for PRs).
+- Search existing issues before creating a new one.
+
+## Pull Request Conventions
+
+### Title Format
+
+```
+GH-<issue-number>: [Component] Short description
+```
+
+Example: `GH-14736: [C++][Python] Ensure no validity bitmap in UnionArray::SetData`
+
+### MINOR Changes
+
+For trivial documentation or typo fixes affecting no more than 2 files and
+no more than 500 words, you may skip creating an issue and prefix the PR
+title with `MINOR:` instead:
+
+```
+MINOR: [Docs] Fix typo in memory.rst
+```
+
+Do NOT combine `MINOR:` with a `GH-` number.
+
+### PR Description
+
+Fill in all four sections of the PR template:
+
+1. **Rationale for this change** - Why is this needed?
+2. **What changes are included in this PR?** - What did you do?
+3. **Are these changes tested?** - How are they validated?
+4. **Are there any user-facing changes?** - Yes/No with brief explanation
+
+Descriptions should be concise and to the point — do not restate things
+that are obvious from reading the diff.
+
+Only keep the "Critical Fix" or "Breaking changes" flags in the description
+if they genuinely apply. Remove them otherwise.
+
+## Pre-commit Hooks
+
+```bash
+pip install pre-commit
+pre-commit install
+pre-commit run -a                        # all hooks
+pre-commit run --all-files <component>   # single component (alias from .pre-commit-config.yaml)
+```
+
+Always run the relevant pre-commit checks before submitting a PR.
+
+## Testing
+
+Run relevant tests locally before submitting a PR. See the
+[Testing](https://arrow.apache.org/docs/developers/guide/step_by_step/testing.html)
+section of the contributor guide for component-specific instructions.
+
+## AI-Generated Code Policy
+
+See: https://arrow.apache.org/docs/dev/developers/overview.html#ai-generated-code
+
+## Component-Specific Guides
+
+Some subdirectories may contain their own `AGENTS.md` with language-specific
+build instructions, testing commands, and coding conventions. Check the
+relevant component directory before starting work.
+
+## Further Resources
+
+- [New Contributor's Guide](https://arrow.apache.org/docs/developers/guide/index.html)
+- [Contributing Overview](https://arrow.apache.org/docs/developers/overview.html)
+- [CONTRIBUTING.md](CONTRIBUTING.md)