Add repository guidelines for Codex Agents

Author: kosiew

AGENTS.md

@@ -0,0 +1,32 @@
+# Repository Guidelines for Codex Agents
+
+This repository uses Rust and contains documentation written in Markdown and TOML configuration files. When modifying code or docs, please follow these steps before committing:
+
+1. **Run the Rust lints**
+   ```bash
+   ./dev/rust_lint.sh
+   ```
+   This script checks formatting (`cargo fmt`), lints (`cargo clippy`), validates `taplo` formatting and generates docs with warnings enabled.
+
+2. **Format Markdown and TOML**
+   ```bash
+   # Format Markdown files
+   prettier -w {datafusion,datafusion-cli,datafusion-examples,dev,docs}/**/*.md
+
+   # Check TOML formatting
+   taplo format --check
+   ```
+
+3. **Run the tests**
+   ```bash
+   cargo test
+   ```
+   You may also use `cargo nextest run` if installed.
+
+4. **Optional: build documentation**
+   ```bash
+   ./docs/build.sh
+   ```
+   This renders the user and contributor guides.
+
+Ensure these commands succeed before creating commits.