[Python] Add AGENTS.md

[thisisnic]

Describe the enhancement requested

Add AGENTS.md

Component(s)

Python

[ybapat]

I've opened a PR for this: #50626

The python/AGENTS.md covers:

• Project overview and Cython architecture
• AI disclosure requirement
• Cython source files and type stubs (hand-written, not generated)
• Development setup via conda
• Building Arrow C++ + PyArrow in editable mode
• Testing with pytest, including --enable-<group> / --disable-<group> flags
• Code style via pre-commit run --all-files python
• Type checking with mypy, pyright, and ty
• File structure reference table
• PR conventions

Happy to adjust anything based on feedback!