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/collab/src/tests/integration_tests.rs

@@ -6,13 +6,15 @@ use crate::{
     },
 };
 use anyhow::{anyhow, Result};
-use assistant::{ContextStore, PromptBuilder, SlashCommandWorkingSet};
+use assistant::ContextStore;
+use assistant_slash_command::SlashCommandWorkingSet;
 use assistant_tool::ToolWorkingSet;
 use call::{room, ActiveCall, ParticipantLocation, Room};
 use client::{User, RECEIVE_TIMEOUT};
 use collections::{HashMap, HashSet};
 use fs::{FakeFs, Fs as _, RemoveOptions};
 use futures::{channel::mpsc, StreamExt as _};
+use prompt_library::PromptBuilder;
 
 use git::status::{FileStatus, StatusCode, TrackedStatus, UnmergedStatus, UnmergedStatusCode};
 use gpui::{