docs: document package KV cache effects

packages/spill/spill-local/README.md

@@ -22,6 +22,8 @@ Files land at `<root>/session-<hash>/​<random>-<safeName>`:
 
 Indirectly, through spill consumers that render the local path and `read`/`grep` retrieval guidance.
 
+**KV Cache effect**: No direct invalidation; the named consumer owns any request-prefix changes.
+
 ## Known Limitations and Deferred Work
 
 - **Local spill files persist until external cleanup** — the backend has no session-lifecycle deletion or age-based retention policy, because persisted, resumed, and forked sessions may still reference a path.

packages/spill/spill-policy/README.md

@@ -40,6 +40,8 @@ The policy sees only the FINAL formatted tool result — not a tool's internal r
 
 **Token effect**: A successful replacement is at most `maxInlineBytes` UTF-8 bytes and remains in history until compaction; the full spill text is not resent to the model.
 
+**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
 
 - **Only final plain-text results are spillable** — mixed-content results, blocked feedback, and `read` pass through; provider truncation or tool-owned retention that happened earlier cannot be recovered here.

packages/spill/spill/README.md

@@ -30,6 +30,8 @@ See the [tool output spill RFC](../../../docs/rfc/implemented/architecture/2026-
 
 Indirectly, through spill consumers that render a backend locator and retrieval guidance.
 
+**KV Cache effect**: No direct invalidation; the named consumer owns any request-prefix changes.
+
 ## Known Limitations and Deferred Work
 
 - **The seam has no retrieval or deletion API** — consumers can only render the backend's locator and guidance; lifecycle and access semantics remain backend-specific.