agent: Rename a number of constructs from Assistant to Agent (#30196)

This PR renames a number of constructs in the `agent` crate from the
"Assistant" terminology to "Agent".

Not comprehensive, but it's a start.

Release Notes:

- N/A

crates/rules_library/src/rules_library.rs

@@ -52,8 +52,8 @@ pub trait InlineAssistDelegate {
         cx: &mut Context<RulesLibrary>,
     );
 
-    /// Returns whether the Assistant panel was focused.
-    fn focus_assistant_panel(
+    /// Returns whether the Agent panel was focused.
+    fn focus_agent_panel(
         &self,
         workspace: &mut Workspace,
         window: &mut Window,
@@ -814,7 +814,7 @@ impl RulesLibrary {
                         .update(cx, |workspace, window, cx| {
                             window.activate_window();
                             self.inline_assist_delegate
-                                .focus_assistant_panel(workspace, window, cx)
+                                .focus_agent_panel(workspace, window, cx)
                         })
                         .ok();
                     if panel == Some(true) {