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_

Start at the /experimental command entry point and the settings.json configuration described in the issue. Trace how the global experimental setting and feature names are currently handled, then verify that show, enable, and disable support independent feature states in both interactive commands and configuration.

索引モデルが issue の本文から書いたものです。