refactor: identify and freeze messages at creation

docs/cookbook/extension-cookbook.md

@@ -40,6 +40,7 @@ A UI plugin renders from the `session/event` feed (the assistant token stream as
 
 ```ts
 import type { Context } from 'cordis'
+import { createUserMessage } from '@deepseek-ai/dsh-llm'
 import { SessionId } from '@deepseek-ai/dsh-session'
 
 declare function render(text: string): void
@@ -54,10 +55,10 @@ export function apply(ctx: Context) {
       render(event.data.chunk.text)
     }
   })
-  onUserInput(text => ctx.agents.get(SessionId('client-session'))?.followup({
+  onUserInput(text => ctx.agents.get(SessionId('client-session'))?.followup(createUserMessage({
     content: [{ type: 'text', text }],
     source: { kind: 'user' },
-  }))
+  })))
 }
 ```