Add an onboarding banner for the Agent panel (#30050)

This PR adds an onboarding banner for the Agent panel:

<img width="262" alt="Screenshot 2025-05-06 at 6 54 58 PM"
src="https://github.com/user-attachments/assets/52849e64-7d5d-488c-8456-4d7bd97f8ebd"
/>

Release Notes:

- N/A

crates/title_bar/src/title_bar.rs

@@ -316,11 +316,11 @@ impl TitleBar {
 
         let banner = cx.new(|cx| {
             OnboardingBanner::new(
-                "Git Onboarding",
-                IconName::GitBranchSmall,
-                "Git Support",
+                "Agentic Onboarding",
+                IconName::ZedAssistant,
+                "Agentic Editing",
                 None,
-                zed_actions::OpenGitIntegrationOnboarding.boxed_clone(),
+                zed_actions::agent::OpenOnboardingModal.boxed_clone(),
                 cx,
             )
         });