fix(apiproxy): derive queued steering on the client, drop it from the wire

The session/queued frame no longer carries steering — AgentMessage no longer
has the field. The client derives it from the same ordered turn boundaries the
host saw (a frame arriving while a turn is open joined the steering FIFO).

packages/host/apiproxy/src/api/events.schema.ts

@@ -36,7 +36,7 @@ export const muxFrameSchema = z.discriminatedUnion('type', [
   z.object({ type: z.literal('question/requested'), sessionId: sessionIdSchema, questions: z.array(askUserQuestionItemSchema).min(1) }),
   z.object({ type: z.literal('question/resolved'), sessionId: sessionIdSchema, questionRpcId: rpcIdSchema, outcome: z.union([z.literal('answered'), z.literal('cancelled')]) }),
   // content/source reuse the wide passthroughs (both are merge-extensible in core).
-  z.object({ type: z.literal('session/queued'), sessionId: sessionIdSchema, content: z.array(contentBlockSchema), source: z.looseObject({ kind: z.string() }), steering: z.boolean() }),
+  z.object({ type: z.literal('session/queued'), sessionId: sessionIdSchema, content: z.array(contentBlockSchema), source: z.looseObject({ kind: z.string() }) }),
   z.object({ type: z.literal('stream/error'), error: rpcErrorSchema }),
 ]) as unknown as z.ZodType<MuxFrame>
 

packages/host/apiproxy/src/api/events.ts

@@ -72,7 +72,7 @@ export type MuxFrame =
    * source carries the prompt's rpcId when the message came over this wire
    * (the client's provisional-echo reconciliation key).
    */
-  | { type: 'session/queued'; sessionId: SessionId; content: ContentBlock[]; source: MessageSource; steering: boolean }
+  | { type: 'session/queued'; sessionId: SessionId; content: ContentBlock[]; source: MessageSource }
   | { type: 'stream/error'; error: RpcError }
 
 /**