[Claude Agent] Add a `/terminal` command that will launch Claude's CLI but using Copilot endpoints

[TylerLeonhardt]

It should be a slash command, but also have a Command Palette command.

We should leverage the same Copilot Messages shim that we use in the Agent for the CLI.

[TylerLeonhardt]

We want to do this in a tasteful way.

The code is present, but we disabled it because we don't want the feature to send the wrong message.