docs(core): define compatibility retention policy

docs/CONFIGURATION_PACKAGES.md

@@ -69,6 +69,11 @@ Required package metadata:
 - migration or transformation rules for older package versions
 - provenance, export source metadata, and signature metadata
 
+Portable configuration schemas follow `COMPATIBILITY_POLICY.md`: providers
+write only their current schema version and read that version plus the previous
+two versions. Older input must produce a version-specific staged-upgrade
+diagnostic.
+
 Configuration fragments are interpreted only by the module that owns them. For
 example, workflow imports workflow definitions; forms imports form schemas;
 mail imports mail templates and delivery defaults; payments imports payment