docs: document package KV cache effects

packages/util/retention/README.md

@@ -85,6 +85,8 @@ const footer = formatRetentionNotice(
 
 Indirectly, through tool consumers that render retained content and omission metadata.
 
+**KV Cache effect**: No direct invalidation; the named consumer owns any request-prefix changes.
+
 ## Known Limitations and Deferred Work
 
 - **Item retention supports `head` only** — tail, head/tail, pagination, grouping, and provider-completeness semantics remain tool-owned.