MCP Selection per Session

### Feature summary

User should be able to select which MCPs to be in the context for a session

### What problem are you trying to solve?

I have many MCPs that can be heavily token consuming (i.e. azure related ones or incident investigation related ones). I do need these MCPs only for a fraction of my sessions. To not mitting max context size, I prefer to keep these off and only turn them on when I need them.

What is currently supported is having MCPs for specific repos only. What I am asking is to have it defined per-session not per-repo.

### Proposed solution

Show a pop-up selection box to the user in the beginning to select which MCPs they want to have.

### Workflow impact

This improves context management and makes users happier with less chance of hitting max context size.

### Installation context

_No response_

### Additional context

_No response_

未指定文件或测试。首先跟踪当前如何按 repository 配置 MCP，以及如何创建 session；然后定义 session 级别的选择流程，并验证只有选定的 MCP 会被包含在 session 上下文中。

由索引模型根据 Issue 内容生成。