Add initial AGENTS.md / CLAUDE.md for coding agents

### Is your feature request related to a problem or challenge?

Arrow Rust already documents development workflows and expectations for AI-assisted contributions in [CONTRIBUTING.md](https://github.com/apache/arrow-rs/blob/main/CONTRIBUTING.md). Adding a repository-level entry point for coding agents would help them discover and follow these instructions consistently.

The goal is to reduce repeated setup work, avoidable CI failures, and reviewer time spent on contributions that miss existing project conventions.

### Describe the solution you'd like

Add a small root `AGENTS.md`, with `CLAUDE.md` symlinked to it, following DataFusion’s approach.

The initial file should:

- Link to `CONTRIBUTING.md`, the repository and crate READMEs, and the PR template.
- Include essential setup and validation commands directly: initialize test-data submodules, run formatting, Clippy, spell checking, and relevant crate tests.
- Explain how to find additional checks and feature combinations in the existing CI workflows, distinguishing checks for documentation-only changes from code changes.
- Point to the existing guidance for benchmarks, Miri, and breaking API changes.
- Surface the existing expectations for AI-generated submissions, including contributor ownership, disclosure, focused changes, and concise descriptions.

Keep detailed guidance in the contributor documentation and improve it where needed. Start small and refine the agent instructions based on recurring problems observed in practice.

### Additional context

Inspired by DataFusion’s [initial issue](https://github.com/apache/datafusion/issues/20941), [documentation follow-up](https://github.com/apache/datafusion/pull/20970), [explicit command guidance](https://github.com/apache/datafusion/pull/21171), and [expanded local validation instructions](https://github.com/apache/datafusion/pull/23313).

Start by reading CONTRIBUTING.md, the repository and crate READMEs, the PR template, and the existing CI workflows. Add a small root AGENTS.md with the requested links, setup and validation commands, CI guidance, and AI-contribution expectations, then symlink CLAUDE.md to it; done means the documented checks and links cover the requested workflow without duplicating detailed guidance.

Written by the indexing model from the issue text.