The single-line `class X: ...` exception stubs tripped
github-code-quality's "Statement has no effect" rule: a bare `...` is a
discarded expression statement. Replace each with a one-line docstring,
matching the empty-error convention used elsewhere (GoalWriteConflict,
ConflictError) and silencing the false positive. No behavior change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>