sidebar: Display a new thread button in the archive view (#59114)

Sometimes, landing in the archive view at first, particularly after
importing ACP threads, can be confusing as there's not an immediately
available "create new thread" button users can click on.

Release Notes:

- N/A

crates/agent_ui/src/threads_archive_view.rs

@@ -133,6 +133,7 @@ pub enum ThreadsArchiveViewEvent {
     Activate { thread: ThreadMetadata },
     CancelRestore { thread_id: ThreadId },
     Import,
+    NewThread,
 }
 
 impl EventEmitter<ThreadsArchiveViewEvent> for ThreadsArchiveView {}
@@ -961,6 +962,14 @@ impl ThreadsArchiveView {
             .child(
                 h_flex()
                     .gap_1()
+                    .child(
+                        IconButton::new("new-thread", IconName::Plus)
+                            .icon_size(IconSize::Small)
+                            .tooltip(Tooltip::text("Start New Agent Thread"))
+                            .on_click(cx.listener(|_this, _, _, cx| {
+                                cx.emit(ThreadsArchiveViewEvent::NewThread);
+                            })),
+                    )
                     .child(
                         IconButton::new("thread-import", IconName::Download)
                             .icon_size(IconSize::Small)

crates/sidebar/src/sidebar.rs

@@ -7672,6 +7672,12 @@ impl Sidebar {
                 ThreadsArchiveViewEvent::Import => {
                     this.show_thread_import_modal("thread_history", window, cx);
                 }
+                ThreadsArchiveViewEvent::NewThread => {
+                    this.show_thread_list(window, cx);
+                    if let Some(workspace) = this.active_workspace(cx) {
+                        this.create_new_entry(&workspace, window, cx);
+                    }
+                }
             },
         );