GH-50341: [C++] Add AGENTS.md to C++ dir

[AdvancedUno]

Rationale for this change

The Arrow C++ library is the core of the monorepo that many language bindings depend on, but there is no AI-agent-oriented development guide for the cpp/ directory. This adds one, mirroring the proposed r/AGENTS.md (#50343), giving agents and new contributors a concise, verified entry point for building, testing, and contributing to Arrow C++.

What changes are included in this PR?

Adds a single new documentation file, cpp/AGENTS.md, covering project overview, AI disclosure, generated files, building (CMake presets), testing (GoogleTest/ctest), code style and linting (clang-format/cpplint using pre-commit), the file-structure layout, and the PR/git workflow.

Are these changes tested?

No, just document

Are there any user-facing changes?

No

• GitHub Issue: [C++] Add AGENTS.md to C++ dir #50341

[github-actions]

⚠️ GitHub issue #50341 has been automatically assigned in GitHub to PR creator.

[thisisnic]

I'm going to close this as it's going to be better if someone with experience of working on Arrow C++ does this, but if you're looking for somewhere to get involved, there are loads of tickets marked "Needs Champion" here that we'd love contributions for!

https://github.com/apache/arrow/issues?q=state%3Aopen%20label%3A%22Status%3A%20needs%20champion%22

[thisisnic]

@AdvancedUno - Changed my mind; if you like, we can work on this collaboratively actually - given you've now made a C++ PR, you're better placed to know what would be good to go in here as a contributor.

The only thing I'll say is that it'll be better if we figure out what we want to put in and let AI polish it, but not have AI generate it in its entirety. I started that R AGENTS.md PR earlier, but I've marked it as a draft as I want to give it some human thought.

How does that sound to you? Also OK if you've moved on to something new and would rather not thought!