Experimental feature flag toggles

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

_No response_

### Proposed solution

I’m using Copilot CLI 1.0.32 on Windows.

I’d like clearer control and documentation for experimental feature flags, especially PERSISTED_PERMISSIONS.

Today `/experimental` lists individual feature flags such as `PERSISTED_PERMISSIONS`, but the public docs and local CLI help only document the global
experimental switch (`--experimental`, `--no-experimental`, `/experimental on|off`).

This makes it unclear whether these flags are:
- individually toggleable
- informational only
- persisted somewhere in config
- or automatically gated by version/account/platform

`PERSISTED_PERMISSIONS` is the experimental feature I need most because repeated permission prompts cost a lot of time.

Please either:
1. document exactly how to enable/disable each feature flag individually, especially `PERSISTED_PERMISSIONS`, or
2. if per-feature toggling is not supported, make that explicit in `/experimental` and in the docs.

It would also help to document how `PERSISTED_PERMISSIONS` behaves:
- what exactly is persisted
- whether it is per repo, per folder, or per location
- where approvals are stored
- how it interacts with session-scoped approvals
- what the user should expect in the permission UI when it is active

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start with the `/experimental` command and the local CLI help, then compare them with the public documentation. Determine whether flags such as `PERSISTED_PERMISSIONS` can be toggled individually and document their persistence, scope, storage, session interaction, and permission-UI behavior; if not, make that limitation explicit.

索引モデルが issue の本文から書いたものです。