Copilot Chat (Agents): folder picker defaults to alphabetically first repository in multi-repo workspace, no way to prefer workspace root

## Problem

In a workspace rooted at a single folder (not multi-root), where Git submodules are detected as additional repositories, starting a new Copilot Chat/Agents session shows a folder/repository picker. The picker appears to sort entries alphabetically and selects the first item by default.

In my case, that means a submodule (e.g. `ui/cengage`) is selected by default instead of the workspace root repo (`pim-www`).

## Why this is an issue

I want to keep submodule awareness enabled, but I still want new chat sessions to default to the workspace root repository unless I explicitly choose a submodule.

Currently, there seems to be no setting to do that.

## Repro

1. Open a repository folder as workspace root (example: `pim-www`).
2. Ensure submodule detection is enabled (`git.detectSubmodules: true`).
3. Have multiple detected repos (root + submodules).
4. Open a new Copilot Chat/Agents session.
5. Observe folder/repo picker default selection.

## Actual behavior

- Repo list appears sorted alphabetically.
- Default selection is the first entry (often a submodule), not workspace root.

## Expected behavior

One of:
1. Default to workspace root repo for new sessions, or
2. Remember last selected repo/folder per workspace, or
3. Provide a setting to control default selection strategy, e.g.:
- `workspaceRoot`
- `lastUsed`
- `alphabeticalFirst` (current behavior)

## What I already tried

- `git.autoRepositoryDetection: "openEditors"`
- `git.repositoryScanMaxDepth: 0`
- `git.scanRepositories: ["."]`

These did not change picker default behavior.

## Environment

- VS Code: 1.102.0
- Copilot Chat: Copilot CLI runtime in VS Code (Agents mode)
- OS: macOS
- Workspace type: single-root folder with Git submodules detected