feat: add a close button to dialog

The close affordance reuses Button (ghost, small) at the panel's top
right and routes through on_change like escape and scrim clicks; it is
hidden in alert mode. The dialog feature now depends on button, and
AGENTS.md codifies preferring existing components over custom elements.

crates/core/tests/dialog.rs

@@ -158,3 +158,26 @@ fn clicking_the_panel_does_not_dismiss() {
     click(&mut visual, point(px(960.), px(540.)));
     assert!(changes.borrow().is_empty());
 }
+
+/// Clicking the close button reports close like escape and the scrim; the
+/// panel then unmounts the button with the rest of the dialog.
+#[test]
+fn the_close_button_reports_close_and_disappears() {
+    let (_cx, mut visual, changes) = dialog_window(true, false);
+
+    let bounds = visual
+        .debug_bounds("dialog-close")
+        .expect("the close button rendered");
+    click(&mut visual, bounds.center());
+    assert_eq!(*changes.borrow(), vec![false]);
+
+    draw(&mut visual);
+    assert!(visual.debug_bounds("dialog-close").is_none());
+}
+
+/// An alert answers only through its actions, so it renders no close button.
+#[test]
+fn an_alert_hides_the_close_button() {
+    let (_cx, mut visual, _changes) = dialog_window(true, true);
+    assert!(visual.debug_bounds("dialog-close").is_none());
+}