GH-50342: [Python] Add AGENTS.md

[ybapat]

Rationale for this change

Add an AGENTS.md file to the python/ directory to help AI agents and new contributors understand how to build, test, and contribute to PyArrow effectively. This follows the pattern established by the root AGENTS.md (GH-50373) and the R AGENTS.md (GH-49607).

Note: This PR was created with the assistance of an AI tool (Claude). The change consists solely of documentation.

What changes are included in this PR?

Adds python/AGENTS.md covering:

• Project overview (Cython-based Python bindings for Arrow C++)
• AI disclosure requirement
• Explanation of Cython source files and type stubs (hand-written vs. generated)
• Development setup via conda
• Building Arrow C++ and PyArrow (editable mode)
• Running tests with pytest, including optional feature group flags (--enable-<group> / --disable-<group>)
• Code style via pre-commit (python alias)
• Type checking with mypy, pyright, and ty
• File structure table
• PR title format and description template
• Links to further resources

Are these changes tested?

No — this is a documentation-only change.

Are there any user-facing changes?

No.

• GitHub Issue: [Python] Add AGENTS.md #50342

[github-actions]

⚠️ GitHub issue #50342 has been automatically assigned in GitHub to PR creator.