feat: make agent switching shortcut configurable (#1186)

Adds Cycle Agent to shortcut settings with Tab as the default
Applies custom shortcut in chat input and model selector
Updates help dialog to show saved shortcut values

packages/ui/src/lib/shortcuts.ts

@@ -297,6 +297,13 @@ const SHORTCUT_ACTIONS: ReadonlyArray<ShortcutAction> = [
     label: 'Cycle thinking variant',
     description: 'Cycle thinking variant while in chat',
   },
+  {
+    id: 'cycle_agent',
+    defaultCombo: 'tab',
+    label: 'Cycle agent',
+    description: 'Cycle agent while the model selector is open',
+    customizable: true,
+  },
   {
     id: 'cycle_favorite_model_forward',
     defaultCombo: 'ctrl+]',