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.

src/wrenn/exceptions.py

@@ -0,0 +1,53 @@
+from __future__ import annotations
+
+
+class WrennError(Exception):
+    """Base exception for all Wrenn SDK errors."""
+
+    def __init__(self, code: str, message: str, status_code: int) -> None:
+        self.code = code
+        self.message = message
+        self.status_code = status_code
+        super().__init__(message)
+
+
+class WrennValidationError(WrennError):
+    """400 — Invalid request parameters."""
+
+
+class WrennAuthenticationError(WrennError):
+    """401 — Invalid or missing authentication."""
+
+
+class WrennForbiddenError(WrennError):
+    """403 — Authenticated but not authorized."""
+
+
+class WrennNotFoundError(WrennError):
+    """404 — Resource not found."""
+
+
+class WrennConflictError(WrennError):
+    """409 — State conflict (e.g. invalid_state)."""
+
+
+class WrennHostHasSandboxesError(WrennConflictError):
+    """409 — Host still has running sandboxes."""
+
+    def __init__(
+        self, code: str, message: str, status_code: int, sandbox_ids: list[str]
+    ) -> None:
+        self.sandbox_ids = sandbox_ids
+        super().__init__(code, message, status_code)
+
+
+class WrennHostUnavailableError(WrennError):
+    """503 — No suitable host available."""
+
+
+class WrennAgentError(WrennError):
+    """502 — Host agent returned an error."""
+
+
+class WrennInternalError(WrennError):
+    """500 — Unexpected server error."""