Copilot Chat (Agents): allow starting a new chat with no workspace folder context

## Problem

Starting a new Copilot Chat/Agents session from the folder picker currently requires selecting a local folder/repository.

There is no way in that flow to start a chat with no workspace context (global/empty-window style).

## Repro Steps

1. Start a new Copilot Chat/Agents session.
2. In the folder/repository picker, try to continue without selecting a folder.

## Actual Behavior

A folder selection is required.

## Expected Behavior

One of the following should be available:

1. A No folder option in the picker.
2. A Start without workspace context toggle.
3. A command/setting to default new Agents chats to no-folder mode.

## Related

Adjacent to, but distinct from, default folder selection behavior in #326520.

## Environment

- VS Code: 1.102.0
- Copilot Chat: Agents mode
- OS: macOS

Start from the Copilot Chat/Agents folder or repository picker flow and compare it with existing no-workspace or empty-window behavior. Review related issue #326520 for adjacent folder-selection behavior; done means a new Agents session can continue without a folder while normal folder-based sessions still work.

Written by the indexing model from the issue text.