Add TUI notifications and attention sounds (disabled by default) (#26980)

packages/opencode/specs/v2/notifications.md

@@ -0,0 +1,13 @@
+# TUI Notifications Default
+
+Problem:
+
+- v1 defaults `attention.enabled` to `false`
+- users can opt in with `attention.enabled = true`
+- v2 should make core TUI notifications a default behavior
+
+## v2 Target
+
+Flip `attention.enabled` to `true` by default in v2.
+
+Keep `attention.enabled = false` as the explicit opt-out.