20.30I'd put a compact rules file in the repo

Something like:

INTEGRATION ENGINEERING RULES

1. Never invent business requirements.
2. Every observable business behavior must trace to BR-*.
3. Every Salesforce write must have an explicitly documented identity/key.
4. Prefer Upsert with stable unique External ID where semantics permit.
5. Never implement Query→Create for duplicate prevention without
   concurrency analysis.
6. Distinguish omitted fields from explicit nulls.
7. Never add a retry without documenting idempotency.
8. Treat timeouts as ambiguous outcomes.
9. Never assume cross-system rollback.
10. Prefer bulk operations over N+1 calls.
11. Never log secrets or unapproved full payloads.
12. Every external dependency needs timeout and error semantics.
13. Async flows require retry, replay/DLQ and lag observability.
14. Every requirement change must trigger test-impact analysis.
15. Mocks cannot prove real Salesforce schema/permissions.
16. Do not declare integration complete based only on HTTP success;
    verify business target state where appropriate.
17. Flag uncertainty instead of guessing.

If your coding AI follows these seventeen rules, it will already behave better than a surprising amount of manually written integration code.