Copilot CLI install blocks the FRE behind a modal spinner

[hamza-usmani]

Description of the new feature

When the Copilot CLI is not already present, the FRE installs it inline during Save via winget, with a 20 minute timeout. On a slow network the user sits in front of a modal spinner and cannot use the terminal at all until it finishes. See the needsCopilot path in FreOverlay.cpp around line 1754.

The install itself is fine. The problem is that it blocks entry to the app.

Proposed technical implementation details

Make the install non-blocking:

• The FRE completes as soon as settings, hooks, and shell integration are written.
• The terminal window opens immediately and is fully usable.
• The agent pane shows "Installing GitHub Copilot..." with progress and a cancel option.
• When winget finishes, the pane transitions into the live agent.
• On failure, show an inline retry in the pane rather than sending the user back through the FRE.
• No behavior change when the CLI is already installed.

This just moves the wait out of the modal FRE so users can get into the app.

Related: #277 asks for granular progress text during this same install; that becomes much easier to show once the install lives in the pane. #231 is a user report of the greyed "Setting up..." button looking frozen, which is the symptom of this issue.