Merge remote-tracking branch 'origin/master' into fix/acp-subagent-cwd

# Conflicts:
#	packages/subagent/subagent-acp/README.md

packages/subagent/subagent-acp/README.md

@@ -65,19 +65,35 @@ Keyless tests drive a scripted ACP subprocess over real stdio. The with-key e2e
 
 ### Child-agent request
 
-**What the model sees**: The remote child receives the standalone task content through ACP plus its own process's configured system prompt, tools, and fresh session. It receives no parent conversation. This provider advertises no optional start-time capabilities, so the local service rejects requests for persona, tool filtering, depth enforcement, or structured output instead of silently omitting them.
+#### What the model sees
 
-**Token effect**: The child pays for an independent full context and its own multi-step history. These tokens never enter the parent's context.
+The remote child receives the standalone task content through ACP plus its own process's configured system prompt, tools, and fresh session. It receives no parent conversation. This provider advertises no optional start-time capabilities, so the local service rejects requests for persona, tool filtering, depth enforcement, or structured output instead of silently omitting them.
+
+#### 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>`.
+#### What the model sees
 
-**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.
+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)).
+- **A fresh process per run** — persistent-process pooling is a future optimization ([the seam Agent Note](../../../.agents/notes/implemented/feature/2026-06-21-subagent-capability-seam.md)).
 - **Local workspaces only** — the resolved cwd is a local path handed to a child on the same machine; workspace mapping for a remote ACP agent would need its own backend capability and is not designed here.
 - **No optional start-time capabilities** — this provider cannot apply the local harness's `outputSchema`, depth cap, tool filter, or persona inside the remote process, so it advertises none and the service rejects requests that require them.
 - **Only `agent_message_chunk` text is collected** — the child's tool-call activity, thought chunks, and plan updates are not surfaced to the parent.