docs: document package KV cache effects

packages/subagent/subagent-acp/README.md

@@ -67,12 +67,16 @@ Keyless tests drive a scripted ACP subprocess over real stdio. The with-key e2e
 
 **Token effect**: The child pays for an independent full context and its own multi-step history. These tokens never enter the parent's context.
 
+**KV Cache effect**: Independent of the parent request cache. Each ACP child can reuse only prefixes identical under its own provider, model, composition, and history; child steps otherwise grow append-only.
+
 ### Parent tool result, indirectly
 
 **What the model sees**: Through `dsh-tool-subagent`, the parent receives only the child's final streamed assistant text or that consumer's exact stop-reason error, not intermediate messages or tool traffic. A request already cancelled before publication becomes exactly `Error: subagent request was aborted before the ACP child started`; other start failures pass through as `Error: <message>`.
 
 **Token effect**: Parent input grows only by the final result or error, which is data-dependent and retained until compaction. This provider adds no parent schema itself.
 
+**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 fresh process per run** — persistent-process pooling is a future optimization ([the seam RFC](../../../docs/rfc/implemented/feature/2026-06-21-subagent-capability-seam.md)).