Allow enabling/disabling individual experimental feature

[devartifex]

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