refactor(agent): unify sourced message delivery

packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts

@@ -87,7 +87,7 @@ describe('startInProcessRun', () => {
 
   it('seeds a forked child but reads only the child-owned output', async () => {
     const { ctx, parent } = await setup([textResponse('parent answer'), textResponse('child answer')])
-    parent.followup([{ type: 'text', text: 'parent question' }])
+    parent.followup({ content: [{ type: 'text', text: 'parent question' }], source: { kind: 'user' } })
     await parent.whenIdle()
     const seed = parent.session.events.slice()
     const run = await startInProcessRun(request(parent), { seed })