- StackAuthError base with code/message/details and from_response() factory
- 13 category subclasses: Authentication, NotFound, Validation, PermissionDenied,
Conflict, OAuth, Passkey, ApiKey, Payment, Email, RateLimit, Cli, Analytics
- Complete _ERROR_CODE_MAP with 143 error codes from known-errors.tsx
- StackAuthModel base class with Pydantic v2 ConfigDict and millis conversion
- Package exports via __init__.py with __all__
- Test StackAuthError base class storage and string representation
- Test all 13 category subclasses inherit from StackAuthError
- Test from_response() dispatch for known and unknown error codes
- Test StackAuthModel config and millis conversion
- Package skeleton with pyproject.toml (stubs only, tests expected to fail)