Add AGENTS.md / CLAUDE.md (#20939)

## Which issue does this PR close?

Closes: #20941
## Rationale for this change

Given that AI tools are used more and more, start with some `AGENTS.md`
and `CLAUDE.md` that simply point to the Contributor Guide /
Architecture Guide.

## What changes are included in this PR?

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## Are these changes tested?

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## Are there any user-facing changes?

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---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Dandandan

AGENTS.md

@@ -0,0 +1,34 @@
+# Agent Guidelines for Apache DataFusion
+
+## Developer Documentation
+
+- [Contributor Guide](docs/source/contributor-guide/index.md)
+- [Architecture Guide](docs/source/contributor-guide/architecture.md)
+
+## Before Committing
+
+Before committing any changes, you **must** run the following checks and fix any issues:
+
+```bash
+cargo fmt --all
+cargo clippy --all-targets --all-features -- -D warnings
+```
+
+- `cargo fmt` ensures consistent code formatting across the project.
+- `cargo clippy` catches common mistakes and enforces idiomatic Rust patterns. All warnings must be resolved (treated as errors via `-D warnings`).
+
+Do not commit code that fails either of these checks.
+
+## Testing
+
+Run relevant tests before submitting changes:
+
+```bash
+cargo test --all-features
+```
+
+For SQL logic tests:
+
+```bash
+cargo test -p datafusion-sqllogictest
+```

CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md