Allow enabling/disabling individual experimental feature

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

Experimental mode is currently all-or-nothing. I want to enable some experimental features while disabling others, such as Rubber Duck.

### Proposed solution

Add support for per-feature experimental toggles, both interactively and in configuration.

For example:

/experimental show
/experimental enable RUBBER_DUCK_AGENT
/experimental disable RUBBER_DUCK_AGENT
/experimental enable MCP_TASKS

And in settings.json:

{
"experimental": true,
"experimentalFeatures": {
"RUBBER_DUCK_AGENT": false,
"MCP_TASKS": true
}
}

### Example prompts or workflows

_No response_

### Additional context

_No response_

从 /experimental 命令入口点和 issue 中描述的 settings.json 配置开始。追踪当前如何处理全局 experimental 设置和功能名称，然后验证 show、enable 和 disable 是否在交互式命令和配置中都支持各个功能的独立状态。

由索引模型根据 Issue 内容生成。