[Bug]: Agent disappears from sidebar after clearing conversation, with no way to restore it

> This issue was translated automatically.

### Issue Checklist

- [x] I understand that issues are for feedback and problem solving, not for complaining in the comment section, and will provide as much information as possible to help solve the problem.
- [x] My issue is not listed in the [FAQ](https://github.com/CherryHQ/cherry-studio/issues/3881).
- [x] I've looked at **pinned issues** and searched for existing [Open Issues](https://github.com/CherryHQ/cherry-studio/issues), [Closed Issues](https://github.com/CherryHQ/cherry-studio/issues?q=is%3Aissue%20state%3Aclosed), and [Discussions](https://github.com/CherryHQ/cherry-studio/discussions), no similar issue or discussion was found.
- [x] I've filled in short, clear headings so that developers can quickly identify a rough idea of what to expect when flipping through the list of issues. And not "a suggestion", "stuck", etc.
- [x] I've confirmed that I am using the latest version of Cherry Studio.

### Platform

Windows

### Version

v2.0.0-beta.3

### Bug Description

After performing the clear conversation operation on a created agent, the agent disappears from the left agent list (sidebar), making it impossible to quickly switch to or select that agent for conversation.

However, the agent is not deleted — it can still be found and all its configuration information viewed in the Agent Management page, including name, avatar, prompt, knowledge base bindings, etc.

More seriously, users cannot find any obvious way in the UI to manually reactivate the agent — the sidebar has no entry for hidden agents, nor is there a right-click menu option to unhide it. Users report not knowing where to manually reactivate it.

Actual behavior:
- ✅ Conversation content is cleared (as expected)
- ❌ Agent disappears from the left list (unexpected)
- ✅ Agent is still viewable in the Agent Management page (configuration data is intact, not deleted)
- ❌ Users cannot find any obvious way in the UI to manually reactivate the agent

### Steps To Reproduce

1. Open Cherry Studio, go to the Agent Management page, and create a new agent (or use an existing one)
2. Ensure the agent is visible in the sidebar list
3. Have a few rounds of conversation with the agent
4. Click the menu button at the top of the conversation (or right-click the conversation) and select Clear Conversation (Clear Topic)
5. Observe the left agent/conversation list: the agent disappears from the list
6. Go to the Agent Management page: the agent still exists, with all configurations intact

### Expected Behavior

After clearing the conversation (clearing topic/context), the agent should remain in the sidebar list for use. Only the delete agent operation should remove it from the list.

### Relevant Log Output

```shell

```

### Additional Context

Additional notes:

- This issue might occur during the following operation: clicking the clear topic button in the conversation area
- This issue might be related to the rendering/filtering logic of the sidebar list — clearing the conversation might trigger a filter condition where agents without active conversations should not be displayed, but the correct behavior should be that agents appear in the list even without active conversations
- My system: Windows

Severity notes:

User feedback specifically mentioned: not knowing where to manually reactivate. This means:
1. The sidebar does not show a hidden agent entry or collapsed area
2. The right-click menu lacks a "pin to sidebar" or "show" option
3. The Agent Management page does not indicate which agents are hidden
4. Without seeing the workaround, users can almost never recover it on their own — the experience is equivalent to the agent vanishing into thin air

Suggested fixes:
- Root fix: After clearing the conversation, the agent should not disappear from the list. The list displays the agents themselves, not their conversation history.
- Fallback fix: If there is a design reason to hide the agent after clearing the conversation, an entry for "show all agents" or "archived agents" should be added at the bottom of the sidebar, or a prominent "pin to sidebar" toggle should be added to the Agent Management page.

Workaround (not intuitive, hard for most users to discover on their own):
1. Go to the Agent Management page
2. Find the disappeared agent in the list
3. Click the agent → open a new conversation
4. The agent will reappear in the sidebar list

This workaround is not intuitive for most users. There is also no clear toggle button like "show in sidebar" on the Agent Management page.

---

Original Content

### Issue Checklist

- [x] I understand that issues are for feedback and problem solving, not for complaining in the comment section, and will provide as much information as possible to help solve the problem.
- [x] My issue is not listed in the [FAQ](https://github.com/CherryHQ/cherry-studio/issues/3881).
- [x] I've looked at **pinned issues** and searched for existing [Open Issues](https://github.com/CherryHQ/cherry-studio/issues), [Closed Issues](https://github.com/CherryHQ/cherry-studio/issues?q=is%3Aissue%20state%3Aclosed), and [Discussions](https://github.com/CherryHQ/cherry-studio/discussions), no similar issue or discussion was found.
- [x] I've filled in short, clear headings so that developers can quickly identify a rough idea of what to expect when flipping through the list of issues. And not "a suggestion", "stuck", etc.
- [x] I've confirmed that I am using the latest version of Cherry Studio.

### Platform

Windows

### Version

v2.0.0-beta.3

### Bug Description

当对一个已创建的智能体（Agent）执行清空会话操作后，该智能体会从左侧的智能体列表（侧边栏）中消失，无法再快捷切换或选择该智能体进行对话。

但是，该智能体并未被删除——在智能体管理（Agent Management）页面中仍然可以找到并查看该智能体的所有配置信息，包括名称、头像、提示词、知识库绑定等。

更严重的是，用户在界面上找不到任何明显的方式手动重新激活该智能体——侧边栏没有已隐藏的智能体入口，也没有取消隐藏的右键菜单选项。用户反馈不知道从哪里手动重新激活。

实际行为：
- ✅ 会话内容被清空（符合预期）
- ❌ 智能体从左侧列表中消失（不符合预期）
- ✅ 智能体在智能体管理页面中仍可查看（配置数据完整，未被删除）
- ❌ 用户在界面上找不到任何明显的方式手动重新激活该智能体

### Steps To Reproduce

1. 打开 Cherry Studio，进入智能体（Agent）管理页面，创建一个新的智能体（或使用已有智能体）
2. 确保该智能体在侧边栏列表中可见
3. 与该智能体进行若干轮对话
4. 点击对话顶部的菜单按钮（或右键会话），选择清空会话（Clear Conversation / 清空话题）
5. 观察左侧智能体/会话列表：该智能体从列表中消失
6. 进入智能体管理页面：该智能体仍然存在，所有配置完好

### Expected Behavior

清空会话（清空话题/清除上下文）后，智能体应继续留在侧边栏列表中以供使用。仅有删除智能体操作才应将其从列表中移除。

### Relevant Log Output

```shell

```

### Additional Context

补充说明：

- 该问题可能发生在以下操作中：点击对话区域的清空话题按钮
- 该问题可能与侧边栏列表的渲染/过滤逻辑有关——清空会话可能触发了某种无会话的智能体不应显示的过滤条件，但正确行为应当是智能体即使没有活跃会话也应显示在列表中
- 我使用的系统：Windows

严重程度补充：

用户反馈中特别提到：不知道从哪里手动重新激活。这意味着：
1. 侧边栏没有显示隐藏的智能体入口或折叠区域
2. 右键菜单没有固定到侧边栏或显示选项
3. 智能体管理页面也没有标注哪些智能体被隐藏了
4. 用户如果没有看到临时方案，几乎无法自行找回——体验上等同于智能体凭空消失了

建议修复方案：
- 根本修复：清空会话后，智能体不应从列表中消失。列表显示的是智能体本身，而非其会话记录。
- 兜底修复：如果确有设计原因需要在清空会话后隐藏该智能体，则应在侧边栏底部增加显示所有智能体或已归档智能体的入口，或在智能体管理页面增加醒目的固定到侧边栏开关。

临时解决方案（不直观，大多数用户难以自行发现）：
1. 进入智能体管理（Agent Management）页面
2. 在列表中找到消失的智能体
3. 点击该智能体 → 打开新会话
4. 智能体会重新出现在侧边栏列表中

此方案对大多数用户并不直观。智能体管理页面上也没有显示在侧边栏之类的清晰开关按钮。