fix(subagent): complete output selection contract

packages/scaffold/server/tests/built-scope-carrier.e2e.ts

@@ -106,8 +106,8 @@ describe.skipIf(!existsSync(jsonrpcBundle))('dsh-jsonrpc BUILT scope carrier', (
     })
 
     expect(stderr).not.toContain('listener threw')
-    // A childless result carries NO lastAssistantMessage on the wire: the end
-    // edge encodes "no output" as an absent field, never `[]`.
+    // A result without output omits lastAssistantMessage from the wire; it
+    // never sends `[]`.
     expect(JSON.parse(stdout) as unknown).toEqual([{
       method: 'subagent.finished',
       params: {

packages/scaffold/server/tests/server.spec.ts

@@ -736,8 +736,8 @@ describe('HarnessSdkServer', () => {
         stopReason: 'error',
       })
 
-      // A childless result carries NO lastAssistantMessage on the wire: the
-      // end edge encodes "no output" as an absent field, never `[]`.
+      // A result without output omits lastAssistantMessage from the wire; it
+      // never sends `[]`.
       expect(transport.notifications).toContainEqual({
         method: 'subagent.finished',
         params: {