Show notifications on primary screen by default (#27665)

By default, agent notifications now display only on your primary screen.
You can optionally configure them to display on all screens (or not to
display at all).

Release Notes:

- N/A

assets/settings/default.json

@@ -670,8 +670,13 @@
         }
       }
     },
-    // Shows a notification when the agent needs confirmation before running an edit tool call or when that's concluded.
-    "notify_when_agent_waiting": true
+    // Where to show notifications when an agent has either completed
+    // its response, or else needs confirmation before it can run a
+    // tool action.
+    // "primary_screen" - Show the notification only on your primary screen (default)
+    // "all_screens" - Show these notifications on all screens
+    // "never" - Never show these notifications
+    "notify_when_agent_waiting": "primary_screen"
   },
   // The settings for slash commands.
   "slash_commands": {