fix(apiproxy): echo the resolved agent preset on create and session-added

session.create's reply named only the id, and the session-added frame's
agentPreset — which the server already derives via sessionListFields — was
stripped by the wire schema it never joined. A client therefore could not
label a session it just created (or learned live) until the next full list
refresh: the header's preset label rendered nothing for exactly the sessions
made in this tab. The create reply is the commit point that knows the
RESOLVED composition (a caller that named none gets the default the header
recorded), so both carriers now say it.

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

@@ -73,6 +73,7 @@ export const hostFrameSchema = z.discriminatedUnion('type', [
     parentSessionId: sessionIdSchema.optional(),
     origin: z.literal('subagent').optional(),
     cwd: z.string().optional(),
+    agentPreset: z.string().optional(),
   }),
   z.object({ type: z.literal('host/session-removed'), sessionId: sessionIdSchema }),
   z.object({ type: z.literal('host/session-status'), sessionId: sessionIdSchema, running: z.boolean() }),

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

@@ -116,6 +116,7 @@ export type HostFrame =
     parentSessionId?: SessionId
     origin?: 'subagent'
     cwd?: string
+    agentPreset?: string
   }
   | { type: 'host/session-removed'; sessionId: SessionId }
   | { type: 'host/session-status'; sessionId: SessionId; running: boolean }

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

@@ -109,6 +109,7 @@ export const sessionCreateRequestSchema = z.object({
 /** session.create response value. */
 export const sessionCreateValueSchema = z.object({
   sessionId: sessionIdSchema,
+  agentPreset: z.string().optional(),
 }) satisfies z.ZodType<Wire<ResponseValue<'session.create'>>>
 
 /** session.rename request payload (raw title; host-side normalization decides acceptance). */

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

@@ -217,7 +217,7 @@ export interface SessionsApi {
    * cannot be mounted fails with `agent-preset-invalid`.
    */
   create(request: RpcRequest<{ workspaceId?: WorkspaceId; cwd?: string; sessionId?: SessionId; agentPreset?: string }>):
-  Promise<RpcResponse<{ sessionId: SessionId }>>
+  Promise<RpcResponse<{ sessionId: SessionId; agentPreset?: string }>>
 
   /**
    * Reads a window of history events; page boundaries align to append-origin message