Enable TUI notifications by default (#6633)

## Summary
- default the `tui.notifications` setting to enabled so desktop
notifications work out of the box
- update configuration tests and documentation to reflect the new
default

## Testing
- `cargo test -p codex-core` *(fails:
`exec::tests::kill_child_process_group_kills_grandchildren_on_timeout`
is flaky in this sandbox because the spawned grandchild process stays
alive)*
- `cargo test -p codex-core
exec::tests::kill_child_process_group_kills_grandchildren_on_timeout`
*(fails: same sandbox limitation as above)*

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_69166f811144832c9e8aaf8ee2642373)

codex-rs/core/src/config/mod.rs

@@ -1323,7 +1323,7 @@ persistence = "none"
     }
 
     #[test]
-    fn tui_config_missing_notifications_field_defaults_to_disabled() {
+    fn tui_config_missing_notifications_field_defaults_to_enabled() {
         let cfg = r#"
 [tui]
 "#;
@@ -1332,7 +1332,7 @@ persistence = "none"
             .expect("TUI config without notifications should succeed");
         let tui = parsed.tui.expect("config should include tui section");
 
-        assert_eq!(tui.notifications, Notifications::Enabled(false));
+        assert_eq!(tui.notifications, Notifications::Enabled(true));
     }
 
     #[test]