Finer plugin control

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

Currenlty all plugins are installed globally at $HOME/.copilot/installed-plugins. Which makes it up the to user to make sure they have disabled at repo level via settings.json. Ideally users should be able to disable all plugins globally, via the global settings.json($HOME/.copilot/settings.json) and just toggle the plugin they need at repo level.

### Proposed solution

_No response_

### Example prompts or workflows

_No response_

### Additional context

_No response_

检查 $HOME/.copilot/installed-plugins 中的插件目录，以及全局和仓库的 settings.json 文件。追踪当前如何确定插件启用状态，然后定义并验证以下行为：全局设置默认禁用插件，而仓库设置选择性地启用插件。

由索引模型根据 Issue 内容生成。