chore: clarify breaking change note in changeset

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: s-adamantine

.changeset/description-union-type.md

@@ -2,4 +2,6 @@
 "@hypercerts-org/lexicon": minor
 ---
 
-Widen description fields on activity, collection, and attachment from a bare Leaflet ref to a union of inline descriptionString and Leaflet linearDocument, with a shared def in org.hypercerts.defs
+Widen description fields on activity, collection, and attachment from a bare Leaflet ref to a union of inline descriptionString and Leaflet linearDocument, with a shared def in org.hypercerts.defs.
+
+This is technically a breaking schema change (ref → union), but not breaking in practice: all published records already include a `$type` discriminator on the description object, so existing data validates against the union without modification.