Add AGENTS.md for AI agent guidance

[harrism]

Summary

Add an AGENTS.md file to the repository root to provide persistent instructions for AI coding agents (Cursor, Copilot, etc.) working on this codebase.

Proposed Content

Git Commits

• All commits must include DCO sign-off (--signoff / -s)
• All commits must be SSH-signed (--gpg-sign / -S)
• Never skip hooks (--no-verify, --no-gpg-sign)

Python Code Style

• Use black with --target-version=py311 --line-length=120 (matches .github/workflows/codestyle.yml)
• Do not run black with default settings (line-length 88)
• Note: a future PR may add [tool.black] to pyproject.toml so plain black works

C++ Code Style

• Use clang-format 18 with the repository's style file for files under src/

License Headers

• Every source file must include the Apache-2.0 SPDX identifier

Testing

• Run relevant tests before pushing: python -m pytest tests/ -v
• The wip/ directory is excluded from test collection

Opening Issues

• Set appropriate labels (e.g. bug, enhancement, documentation)
• Reference any related issues or PRs
• For bugs: provide clear reproduction steps, expected vs actual behavior, and environment details

Opening Pull Requests

• Reference the issue being fixed (e.g. "Fixes #NNN")
• Include a test plan with specific commands to verify the change
• Ensure all CI checks pass before requesting review (DCO, codestyle, tests)
• Keep PRs focused on a single concern