Support opening an Agents Window chat for a folder from the CLI

I'd like to open the Agents Window from the command line and start a new chat scoped to a specific folder.

Proposed usage:
```sh
code-insiders ./foo --agents
```

Expected behavior:
- Open (or focus) the Agents Window.
- Open the New Chat view.
- Use `./foo` as the folder for the new chat.

The equivalent should also work with the stable `code` CLI.