add a basic AGENTS file

Author: arashbm

AGENTS.md

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+# Agent Guidelines for reticula-python
+
+## Build & Test Commands
+- **Build**: `python -m pip install .[test] -v` (builds C++ extension with nanobind)
+- **Test all**: `python -m pytest .`
+- **Test single**: `python -m pytest tests/test_filename.py::test_function_name`
+- **Lint**: `flake8` (extends ignore E402 for import order)
+
+## Project Structure
+- C++ source in `src/` (uses nanobind, CMake, C++20)
+- Python package in `src/reticula/`
+- Tests in `tests/` using pytest + hypothesis
+- Uses scikit-build-core for building
+
+## Code Style
+- **Python**: Follow PEP 8, snake_case naming, type hints preferred
+- **C++**: LLVM style (see .clang-format), 2-space indent, left pointer alignment
+- **Imports**: Relative imports with underscore prefix for internal modules
+- **Generic attributes**: Use `_generic_attribute` pattern for templated functions
+- **Error handling**: Raise appropriate exceptions with descriptive messages
+
+## Key Patterns
+- Generic templated functions via `generic_attribute` metaclass
+- C++ types exposed as Python classes with `__name__` attributes
+- Network types parameterized by vertex and time types
+- Use `_reticula_ext` for C++ bindings, expose through main module