Add AGENTS.md / CLAUDE.md - #20939

#20970)

## Which issue does this PR close?

  - Closes #.

 ## Rationale for this change

This PR is a follow-up to
#20939 from @Dandandan.

The goal is to make it easier for both humans and agents to get started
making changes in this repository and to create pull requests
efficiently.

The repository already had the necessary contributor information, but it
was spread across multiple documents and not easy to discover quickly.

  ## What changes are included in this PR?

This PR makes the most important setup, testing, and pre-PR checks
easier to find from the contributor guide and from `AGENTS.md`:

- Add a quick-start setup section to the contributor guide with the
shortest path to a working local environment.
- Add a testing quick-start section summarizing the most important tests
to run before submitting a PR.
- Add a “Before Submitting a PR” section to centralize formatting and
lint guidance.
- Update `AGENTS.md` to point to the canonical contributor guide
sections instead of duplicating setup and testing instructions.

  ## Are these changes tested?

  This PR updates documentation only.


  ## Are there any user-facing changes?

This improves contributor-facing documentation and makes setup / testing
guidance easier to discover, but it does not change DataFusion runtime
behavior or public APIs.

---------

Co-authored-by: Yongting You <[email protected]>

## Which issue does this PR close?

Closes: apache#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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is sometimes worth providing a summary of the individual changes in this
PR.
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## Are these changes tested?

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2. Serve as another way to document the expected behavior of the code

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## Are there any user-facing changes?

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If there are user-facing changes then we may require documentation to be
updated before approving the PR.
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If there are any breaking changes to public APIs, please add the `api
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---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

apache#20970)

## Which issue does this PR close?

  - Closes #.

 ## Rationale for this change

This PR is a follow-up to
apache#20939 from @Dandandan.

The goal is to make it easier for both humans and agents to get started
making changes in this repository and to create pull requests
efficiently.

The repository already had the necessary contributor information, but it
was spread across multiple documents and not easy to discover quickly.

  ## What changes are included in this PR?

This PR makes the most important setup, testing, and pre-PR checks
easier to find from the contributor guide and from `AGENTS.md`:

- Add a quick-start setup section to the contributor guide with the
shortest path to a working local environment.
- Add a testing quick-start section summarizing the most important tests
to run before submitting a PR.
- Add a “Before Submitting a PR” section to centralize formatting and
lint guidance.
- Update `AGENTS.md` to point to the canonical contributor guide
sections instead of duplicating setup and testing instructions.

  ## Are these changes tested?

  This PR updates documentation only.


  ## Are there any user-facing changes?

This improves contributor-facing documentation and makes setup / testing
guidance easier to discover, but it does not change DataFusion runtime
behavior or public APIs.

---------

Co-authored-by: Yongting You <[email protected]>