refactor: unify agent and session identity

docs/cookbook/extension-cookbook.md

@@ -36,7 +36,7 @@ A UI plugin renders from the `session/event` feed (the assistant token stream as
 
 ```ts
 import type { Context } from 'cordis'
-import { AgentId } from '@deepseek-ai/dsh-agent'
+import { SessionId } from '@deepseek-ai/dsh-session'
 
 declare function render(text: string): void
 declare function onUserInput(handler: (text: string) => void): void
@@ -50,7 +50,7 @@ export function apply(ctx: Context) {
       render(event.data.chunk.text)
     }
   })
-  onUserInput(text => ctx.agents.get(AgentId('main'))?.send([{ type: 'text', text }]))
+  onUserInput(text => ctx.agents.get(SessionId('client-session'))?.send([{ type: 'text', text }]))
 }
 ```