refactor: identify and freeze messages at creation

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

@@ -1,3 +1,4 @@
+import { createUserMessage } from '@deepseek-ai/dsh-llm'
 import { describe, expect, it } from 'vitest'
 import { Context, symbols, type EffectMeta } from 'cordis'
 import Loader from '@cordisjs/plugin-loader'
@@ -106,7 +107,7 @@ 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({ content: [{ type: 'text', text: 'parent prompt' }], source: { kind: 'user' } })
+    parent.followup(createUserMessage({ content: [{ type: 'text', text: 'parent prompt' }], source: { kind: 'user' } }))
     await parent.whenIdle()
     const parentEventCount = parent.session.events.length
     expect(parentEventCount).toBeGreaterThan(0)
@@ -383,7 +384,7 @@ describe('dsh-subagent-spawn', () => {
         textResponse('parent answer'),
         textResponse('child answer'),
       ])
-      parent.followup({ content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } })
+      parent.followup(createUserMessage({ content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }))
       await parent.whenIdle()
 
       const run = await start(ctx, 'spawn', {