condense backend migrations

ai/backend-context.md

@@ -35,6 +35,15 @@ intentionally omitted here - update this section as entities land.
     `new Set(...)`) instead of creating them through their fake
     repositories
 
+## Migrations
+
+- This project is not in production. By default, schema changes should update
+  the relevant create-table migration so migrations describe the current desired
+  schema from scratch.
+- Do not add alter-table or data-backfill migrations unless the user explicitly
+  asks for production-style migration safety.
+- Put seed data in seeders, not migrations.
+
 ## PHP rules
 
 - Imports: always put `use` statements at the top of the file, never use