feat(box): add sandbox_exec tool loop for local-agent calculations

src/langbot/pkg/box/errors.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+
+class BoxError(RuntimeError):
+    """Base error for LangBot Box failures."""
+
+
+class BoxValidationError(BoxError):
+    """Raised when sandbox_exec arguments are invalid."""
+
+
+class BoxBackendUnavailableError(BoxError):
+    """Raised when no supported container backend is available."""
+
+
+class BoxSessionConflictError(BoxError):
+    """Raised when an existing session cannot satisfy a new request."""