docs: document package KV cache effects

packages/core/agent/README.md

@@ -75,12 +75,16 @@ The handle every plugin programs against:
 
 **Token effect**: Accepted content becomes retained history or a repeated session prefix; blocked content contributes no request tokens. Size is caller- and plugin-dependent.
 
+**KV Cache effect**: Accepted history and steering are append-only; a blocked submission sends no request. A session prefix remains stable within its loop instance, while a new or resumed instance may establish a different prefix.
+
 ### Agent-scoped request composition
 
 **What the model sees**: Registrations through `agent.ctx` can shadow prompt sections or tools and can install agent-only interceptors during unpublished setup.
 
 **Token effect**: The package adds zero tokens itself; scoped contributions affect only that agent and disappear on disposal.
 
+**KV Cache effect**: Prefix-stable while an agent's scoped registrations are unchanged. Setup or reload that changes prompt sections, tool definitions, or request listeners may invalidate reuse from the first affected request token.
+
 ## Known Limitations and Deferred Work
 
 - **Initiator scope is process-local** — workers, child processes, HTTP, durable queues, and restarts materialize any required identity explicitly.