[产品讨论] PR #958 feat(desktop): add stdio custom MCP support

## 触发原因

PR #958 新增 stdio 自定义 MCP 支持，属于新增重要产品能力（允许用户接入自定义 MCP 服务）。

## 需要确认

- stdio MCP 的产品定位与目标用户？
- 安全边界与权限模型？
- 与现有 MCP 能力的关系？

## 放行方式

产品白名单成员（@dashhuang / @xdanger / @MagicLizi / @zqchris）在本 issue 或 PR 评论区明确同意。

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关联 PR:#958(作者 @CandyACE);本 issue 由 review-pr 流程自动创建,用于先讨论该 PR 涉及的产品 / UI 变更,聊清楚后 PR 会恢复审查。

Start by reading PR #958 and this issue's product questions about stdio MCP positioning, target users, security boundaries, permissions, and its relationship to existing MCP support. Done means the product discussion is resolved and an approved whitelist member explicitly authorizes the PR to resume review.

Written by the indexing model from the issue text.