agent: Add new thread start buttons to the empty state (#34829)

Release Notes:

- N/A

crates/agent_ui/src/ui.rs

@@ -2,6 +2,7 @@ mod agent_notification;
 mod burn_mode_tooltip;
 mod context_pill;
 mod end_trial_upsell;
+mod new_thread_button;
 mod onboarding_modal;
 pub mod preview;
 mod upsell;
@@ -10,4 +11,5 @@ pub use agent_notification::*;
 pub use burn_mode_tooltip::*;
 pub use context_pill::*;
 pub use end_trial_upsell::*;
+pub use new_thread_button::*;
 pub use onboarding_modal::*;