assistant: Include worktree name in diagnostics slash command (#13354)

Files included with the diagnostics command now include the worktree
name, making it more consistent with the way other commands work
(`/active`, `/tabs`, `/file`). Also, the diagnostics command will now
insert nothing when there are no diagnostics.

Release Notes:

- N/A

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -50,6 +50,7 @@ pub type RenderFoldPlaceholder = Arc<
         + Fn(ElementId, Arc<dyn Fn(&mut WindowContext)>, &mut WindowContext) -> AnyElement,
 >;
 
+#[derive(Default)]
 pub struct SlashCommandOutput {
     pub text: String,
     pub sections: Vec<SlashCommandOutputSection<usize>>,