Add a keyboard shortcut for quick model selection

Feature summary

Add a keyboard shortcut for quick model selection

What problem are you trying to solve?

Switching models should be quick without moving from the keyboard to the mouse.
A dedicated shortcut would make model selection easier to discover and faster than
navigating to the composer's model picker.

Proposed solution

Add a remappable shortcut and a "Switch model..." command in the command palette
that open the model picker with search focused. Support typing to filter, arrow
keys to navigate, Enter to select, and Escape to cancel, then return focus to the
composer.

This should work in both project sessions and standalone chats, preserving the
existing behavior when switching models during a running turn.

Optional enhancement

Allow users to bind shortcuts directly to favorite models for one-step switching.

Workflow impact

Reduces friction when choosing different models for different tasks, including
balancing speed, capability, and cost.

Installation context

GitHub Copilot desktop app; not specific to a repository or organization installation.

Start by locating the desktop app's existing model picker, command palette, composer focus handling, and remappable shortcut registration. Trace how model switching works in project sessions and standalone chats, then verify keyboard filtering, navigation, selection, cancellation, focus restoration, and behavior during a running turn; treat favorite-model bindings as separate optional scope.