Add AGENTS.md file

AGENTS.md

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+# AGENTS.md
+
+Python library for autograding Git repositories, used by Git Mastery exercise repository to verify exercise solutions.
+
+## Dev environment
+
+- Python 3.13+
+- Install dependencies: `pip install -r requirements.txt`
+- Install package in dev mode: `pip install -e .`
+
+## Testing
+
+- Run tests: `python -m pytest -s -vv`
+- Tests are located in `tests/`
+
+## Project structure
+
+- `src/git_autograder/` - Main package
+  - `repo/` - Repository handling (GitAutograderRepo)
+  - `branch.py`, `commit.py`, `remote.py` - Git object wrappers
+  - `exercise.py` - Exercise configuration and grading
+  - `helpers/` - Utility functions
+- `docs/` - Documentation
+- `tests/` - Test suite
+
+## Code style
+
+- **Type hints**: Required on all function signatures. Use `Optional[T]` for nullable types, union syntax `A | B` for alternatives
+- **Docstrings**: Use reStructuredText format with `:param:` and `:type:` tags for public APIs
+- **Imports**: Group in order: stdlib → third-party → local. Use absolute imports from `git_autograder`
+- **Naming**: Classes use `GitAutograder` prefix (e.g., `GitAutograderRepo`). Private methods use single underscore prefix
+- **Exceptions**: Raise `GitAutograderInvalidStateException` for invalid states, `GitAutograderWrongAnswerException` for grading failures
+- **Dataclasses**: Prefer `@dataclass` for simple data containers