Agents window: Remote workspace folders cannot be attached as chat context

- Copilot Chat Extension Version: 0.60.0
- VS Code Version: 1.132.0
- OS Version: macOS arm64 client; WSL2 Linux workspace on a remote Windows host
- Feature: Agents window / Add Context
- Selected model: Any
- Logs: Sanitized patterns included below

## Summary

Folders from a Remote - WSL workspace cannot be attached as chat context in the standalone Agents window.

Dragging a folder from the Agents window's Files explorer into the composer silently does nothing. The alternative **Add Context -> Files & Folders** flow also cannot find folders that are visibly present in the workspace Explorer.

This differs from #301847 because that report used an older VS Code version and concerned a detached Copilot Chat view. The current issue reproduces in the dedicated Agents window on VS Code 1.132.0 and also affects the folder context picker. It may share a remote-resource boundary with #328295, but that issue specifically tracks remote MCP availability.

## Steps to reproduce

1. Connect to a Windows remote host and open a workspace inside WSL using Remote - WSL.
2. Open the standalone **Agents** window for that workspace.
3. In the Agents window's Files explorer, drag a workspace folder into the new-session chat composer.
4. Observe that no context item is added.
5. Select **Add Context -> Files & Folders**.
6. Search for the same folder, which remains visible in Explorer.

## Expected

The folder is attached as context, either by drag-and-drop or through the Files & Folders picker.

## Actual

- Drag-and-drop silently adds nothing.
- The Files & Folders picker returns no matching remote folder.
- The folder remains visible and accessible in Explorer.
- Reloading the VS Code window does not resolve the problem.
- Opening the workspace Chat view and using its Files & Folders picker can show the same failure.

## Sanitized diagnostic pattern

The renderer and agent-host logs repeatedly contain errors shaped like:

```text
[RemoteAgentHostProtocol] Resource not found: file:///home///
[ProtocolServer] resourceResolve failed: file:///home///
```

The diagnostic URI uses a plain `file:` scheme for a resource that belongs to the remote WSL workspace.

## Environment

- VS Code: 1.132.0
- GitHub Copilot Chat: 0.60.0
- Remote - WSL: 0.104.3
- Client OS: macOS arm64
- Remote topology: client -> Windows remote host -> WSL2 Linux workspace
- Reproduces after **Developer: Reload Window**

## Extensions disabled

Not tested with all extensions disabled because the reproduction requires the built-in Agents/Copilot experience and Remote - WSL.