Remote Agent Host artifacts cannot be opened from the local client

## Steps to Reproduce

1. Connect to a remote Agent Host from the Agents window.
2. Start a chat in a folder and use this prompt:

```text
create a temp file test.txt in the vscode folder with some text, then return it to me as an artifact
```

3. Once the session attaches the `test.txt` artifact, click it.

## Actual Result

The artifact refers to a path in the remote Agent Host session filesystem, so opening it from the local client fails.


## Expected Result

Artifacts produced by a remote Agent Host should be transferred or downloaded so they can be opened from the local client.

_Created from a [Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1788068510426229?thread_ts=1788068510.426229&cid=C09MVCZKLP4)._

Start from artifact opening in the Agents window and trace how a remote Agent Host session filesystem path is handled by the local client. Reproduce the reported chat flow, then verify that a remotely produced test.txt artifact is transferred or downloadable and opens successfully locally.

Written by the indexing model from the issue text.