Announce shared connector runtime contract

docs/CONCEPT.md

@@ -25,6 +25,20 @@ Detailed follow-up documents:
 - [OpenProject connector concept](OPENPROJECT_CONNECTOR.md)
 - [OpenDesk integration map](OPENDESK_INTEGRATION_MAP.md)
 
+## Shared runtime boundary
+
+Connector transports use the versioned Core runtime contract for bounded dry
+runs and diagnostics. Connectors owns endpoint discovery, authentication
+hand-off, protocol reads, retry/backoff, and source health. A consuming module
+owns domain mappings and mutations: Addresses, for example, owns contact and
+vCard semantics even when Connectors supplies reusable LDAP, CardDAV, Exchange,
+or Google transport patterns.
+
+Dry runs carry an immutable input hash, source revision and fingerprint,
+redacted effects, diagnostics, truncation state, and an apply token. Apply must
+reject a changed input or source revision. URLs and diagnostics never contain
+credential material; they retain only credential-envelope references.
+
 ## Ownership
 
 The module owns: