feat: implement client architecture and sandbox environment

Introduces the core Wrenn client and a dedicated sandbox execution
environment. This includes automated model generation and a custom
exception hierarchy to support robust integration.

- Add `WrennClient` in `src/wrenn/client.py` for API interaction.
- Implement `Sandbox` in `src/wrenn/sandbox.py` for isolated execution.
- Add Pydantic/model support via `_generated.py`.
- Define project-specific error types in `exceptions.py`.
- Include AGENTS.md documentation for specialized logic.
- Add comprehensive unit and integration tests.
- Update build system (Makefile, uv.lock, pyproject.toml) and LICENSE.

.gitignore

@@ -174,3 +174,4 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 
+CODE_EXECUTION.md