Refactor the legacy invoice module. Don't run database migrations. Don't call any external API with real credentials. Don't push commits or open pull requests. Don't delete, skip, or weaken a test to make it pass. Don't touch any file outside src/billing. Ask before deleting a file. If a step isn't on this list, stop and ask first.
(All the above can be in the prompt, a skill, an agents.md or harness)