refactor(agent): unify sourced message delivery

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

@@ -106,7 +106,7 @@ describe('dsh-subagent-spawn', () => {
   it('a fresh child does NOT inherit the parent conversation (its log starts empty before the prompt)', async () => {
     // Drive the parent through one real turn so it has history, THEN spawn.
     const { ctx, parent } = await setup([textResponse('parent turn'), textResponse('child sees nothing')])
-    parent.followup([{ type: 'text', text: 'parent prompt' }])
+    parent.followup({ content: [{ type: 'text', text: 'parent prompt' }], source: { kind: 'user' } })
     await parent.whenIdle()
     const parentEventCount = parent.session.events.length
     expect(parentEventCount).toBeGreaterThan(0)
@@ -383,7 +383,7 @@ describe('dsh-subagent-spawn', () => {
         textResponse('parent answer'),
         textResponse('child answer'),
       ])
-      parent.followup([{ type: 'text', text: 'hi' }])
+      parent.followup({ content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } })
       await parent.whenIdle()
 
       const run = await start(ctx, 'spawn', {