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/theme/src/styles/players.rs

@@ -129,6 +129,10 @@ impl PlayerColors {
         *self.0.first().unwrap()
     }
 
+    pub fn agent(&self) -> PlayerColor {
+        *self.0.last().unwrap()
+    }
+
     pub fn absent(&self) -> PlayerColor {
         *self.0.last().unwrap()
     }