[FEATURE]: add "/experimental" command to toggle experimental features

[ropl-btc]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Codex CLI has a /experimental commands which makes it very streamlined and easy to toggle experimental features. OpenCode should have that too IMO to make experimental features easier to test and more streamlined.

[github-actions[bot]]

This issue might be a duplicate of existing issues or closely related to them. Please check:

• #7716: Requests adding "/plugins" command to discover Plugins, MCPs, and Skills - very similar intent of adding streamlined slash commands for feature management (like Claude Code)
• #7687: Requests ability to disable plugins via CLI - shares the goal of streamlined feature toggling
• #6719: Requests "/reload" slash command for runtime configuration reloading
• #7679: Requests slash command to show active instruction file paths
• #7466: Discusses toggling MCP features on/off dynamically

Feel free to ignore if your specific use case differs from these existing requests.