chore: add missing changeset for @modelcontextprotocol/core (#2400)

Author: felixweinberger

.changeset/core-subscription-schemas.md

@@ -0,0 +1,10 @@
+---
+'@modelcontextprotocol/core': major
+---
+
+Align the published schema set with the 2026-07-28 surface: removes the
+`RequestMetaEnvelopeSchema` export; adds `SubscriptionFilterSchema`, the
+`SubscriptionsListen*` request/result schemas, and the
+`SubscriptionsAcknowledged*` notification schemas. The bundled schemas now
+match `@modelcontextprotocol/client` and `@modelcontextprotocol/server` of
+the same release.