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/Cargo.toml

@@ -76,7 +76,7 @@ tooltip = []
 menu = ["scroll_area"]
 dropdown_menu = ["menu", "button"]
 select = ["menu"]
-dialog = []
+dialog = ["button"]
 empty_state = []
 alert = []
 toast = []