Allow changing model in chat summary view

We display what model we're using but force users to go to their settings and change it. Would be nice to just allow users to swap their model during the summary phase, and even re-request the initial prompt.

If this feature is added we should also look at a "save as default" button that gives a user a clear indication that will now be the model used and displayed within the extension.

Start by locating the chat summary view and the existing model settings flow in the TypeScript extension. Define the interaction for changing the model during the summary phase, re-requesting the initial prompt, and optionally saving the choice as the default; done means the displayed and used model update consistently.

Written by the indexing model from the issue text.