chore: added AGENTS.md

Author: fstagni

AGENTS.md

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+# DIRAC Agent Guidelines
+
+## Build/Lint/Test Commands
+- **Build**: `pip install -e .`
+- **Lint**: `ruff check src/ && pylint src/`
+- **Test**: `pytest tests/`
+- **Single test**: `pytest src/DIRAC/path/to/test.py::test_function`
+
+## Code Style Guidelines
+- **Formatting**: Use `black` with line length 120 (configured in pyproject.toml)
+- **Imports**: Absolute imports only; sort with `isort` (black profile)
+- **Naming**: CamelCase for classes, snake_case for functions/variables
+- **Types**: Use type hints; run `mypy` for strict checking
+- **Error handling**: Return `S_OK(result)` or `S_ERROR(message)` from DIRAC.Core.Utilities.ReturnValues
+- **Logging**: Use `gLogger.info/warn/error` (from DIRAC import gLogger)
+- **Docstrings**: Follow Google/NumPy style where present
+- **Security**: Never log secrets; validate inputs