/agent - Leave scroll position at top of picklist

### Describe the feature or problem you'd like to solve

As a user, when searching for an agent, I don't want to take my hands off the keyboard to mouse scroll up to the top of the agent selection list

### Proposed solution

Leave the scroll position such that the top of the agent list is displayed, where I can then start a search and see the fiiltered results

### Example prompts or workflows

_No response_

### Additional context

從 /agent picklist 入口開始，檢查代理選擇清單的捲動位置是如何初始化的。完成標準是開啟清單時顯示清單頂部，讓使用者無需手動捲動即可開始使用鍵盤搜尋。

由索引模型根據 Issue 內容生成。