Migration-ready schema — importable from day one

Every SMB product's wedge is low switching cost — the customer brings their data from the incumbent. If the schema can't receive that data idempotently and reversibly, the migration-import-engineer blocks and the wedge is a slogan. These rules are cheap at design time and impossible to retrofit cleanly. Apply them to the data model in ARCH, before any import is designed.

The three rules (the ones that actually block imports)

1. Importable entities carry source_ref (provenance)

Any entity that can be populated from an incumbent export or a third-party sync gets a nullable, unique source_ref column:

source_ref  text  UNIQUE NULL   -- e.g. "servicetitan:pricebook:8842"