docs: document package KV cache effects

packages/ui/stdio/README.md

@@ -29,12 +29,16 @@ The plugin seeds display labels from the live agent registry, then tracks `agent
 
 **Token effect**: Submitted text is retained under the agent loop's normal session-history and compaction rules. The welcome banner, `> ` prompt, rendered transcript, and `[tool call]` / `[tool result]` terminal lines add no tokens.
 
+**KV Cache effect**: Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.
+
 ### Terminal user-interaction answers
 
 **What the model sees**: When a consumer calls `ctx.userInteraction.ask()`, this provider renders the question in the terminal and returns selected option labels or `custom` text. Through `dsh-tool-ask-user`, closed stdin becomes `Error: ask_user_question cannot be answered because stdin is closed`; disposal or abort becomes `Error: ask_user_question was interrupted before the user answered`.
 
 **Token effect**: Waiting and terminal prompts add no tokens; the resolved answer or error is model-visible only through the calling tool or plugin's result.
 
+**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
 
 - **One configured session receives stdin** — the session/event renderer can print output from any session, but input lines always drive the configured `sessionId` rather than routing by the visible label.