CLAUDE.md@eval/multistage_cases/invoice_calc/start git:20261023.6a133eaA

Immutable. This exact content never changes and is served at /api/v1/blob/04602ca1787ef0db.

# Project rules

1. Keep `docs/reference.md` current whenever behaviour changes.
2. Add a `CHANGELOG.md` entry for every change.
3. Every bug fix carries a test.
4. Never edit `src/invoicekit/_generated.py`.