assistant: Remove some re-exports (#23262)

This PR removes some re-exports from the `assistant` to make it clearer
what its constituent modules depend on.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -12,7 +12,6 @@ pub(crate) mod windows_only_instance;
 use anyhow::Context as _;
 pub use app_menus::*;
 use assets::Assets;
-use assistant::PromptBuilder;
 use breadcrumbs::Breadcrumbs;
 use client::{zed_urls, ZED_URL_SCHEME};
 use collections::VecDeque;
@@ -32,6 +31,7 @@ use outline_panel::OutlinePanel;
 use paths::{local_settings_file_relative_path, local_tasks_file_relative_path};
 use project::{DirectoryLister, ProjectItem};
 use project_panel::ProjectPanel;
+use prompt_library::PromptBuilder;
 use quick_action_bar::QuickActionBar;
 use recent_projects::open_ssh_project;
 use release_channel::{AppCommitSha, ReleaseChannel};