docs: document package KV cache effects

packages/ui/tool-ask-user/README.md

@@ -27,12 +27,16 @@ This is the consumer package for the user-interaction seam. It does not render U
 
 **Token effect**: Fixed schema cost on every request where the tool is visible.
 
+**KV Cache effect**: Prefix-stable while the definition and visibility are unchanged. Plugin lifecycle or scoped restrictions may invalidate reuse from this schema.
+
 ### Tool-call history and result
 
 **What the model sees**: The model's full questions remain in the assistant tool-call arguments. After the human answers, the next step sees compact JSON in the exact shape `{"answers":[{"id":"<id>","selected":["<label>"],"custom":"<text>"}]}`; `custom` is omitted when unused and `selected` can contain zero, one, or several labels. UI interaction while the call is pending is not model context.
 
 **Token effect**: Arguments and answer JSON are data-dependent retained tokens; there is no token cost while waiting for the human.
 
+**KV Cache effect**: Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.
+
 ## Known Limitations and Deferred Work
 
 - **A pending question blocks the tool call until the human answers** — the tool declares no `timeout-policy` budget; cancellation rides the turn's `exec.signal` only.