Add the ability to follow the agent as it makes edits (#29839)

Nathan here: I also tacked on a bunch of UI refinement.

Release Notes:

- Introduced the ability to follow the agent around as it reads and
edits files.

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/repl/src/notebook/notebook_ui.rs

@@ -829,7 +829,7 @@ impl ProjectItem for NotebookEditor {
 
     fn for_project_item(
         project: Entity<Project>,
-        _: &Pane,
+        _: Option<&Pane>,
         item: Entity<Self::Item>,
         window: &mut Window,
         cx: &mut Context<Self>,