Extract an agent_ui crate from agent (#33284)

This PR moves the UI-dependent logic in the `agent` crate into its own
crate, `agent_ui`. The remaining `agent` crate no longer depends on
`editor`, `picker`, `ui`, `workspace`, etc.

This has compile time benefits, but the main motivation is to isolate
our core agentic logic, so that we can make agents more
pluggable/configurable.

Release Notes:

- N/A

crates/agent/src/thread_store.rs

@@ -1,22 +1,25 @@
-use std::cell::{Ref, RefCell};
-use std::path::{Path, PathBuf};
-use std::rc::Rc;
-use std::sync::{Arc, Mutex};
-
+use crate::{
+    context_server_tool::ContextServerTool,
+    thread::{
+        DetailedSummaryState, ExceededWindowError, MessageId, ProjectSnapshot, Thread, ThreadId,
+    },
+};
 use agent_settings::{AgentProfileId, CompletionMode};
 use anyhow::{Context as _, Result, anyhow};
 use assistant_tool::{ToolId, ToolWorkingSet};
 use chrono::{DateTime, Utc};
 use collections::HashMap;
 use context_server::ContextServerId;
-use futures::channel::{mpsc, oneshot};
-use futures::future::{self, BoxFuture, Shared};
-use futures::{FutureExt as _, StreamExt as _};
+use futures::{
+    FutureExt as _, StreamExt as _,
+    channel::{mpsc, oneshot},
+    future::{self, BoxFuture, Shared},
+};
 use gpui::{
     App, BackgroundExecutor, Context, Entity, EventEmitter, Global, ReadGlobal, SharedString,
-    Subscription, Task, prelude::*,
+    Subscription, Task, Window, prelude::*,
 };
-
+use indoc::indoc;
 use language_model::{LanguageModelToolResultContent, LanguageModelToolUseId, Role, TokenUsage};
 use project::context_server_store::{ContextServerStatus, ContextServerStore};
 use project::{Project, ProjectItem, ProjectPath, Worktree};
@@ -25,19 +28,18 @@ use prompt_store::{
     UserRulesContext, WorktreeContext,
 };
 use serde::{Deserialize, Serialize};
-use ui::Window;
-use util::ResultExt as _;
-
-use crate::context_server_tool::ContextServerTool;
-use crate::thread::{
-    DetailedSummaryState, ExceededWindowError, MessageId, ProjectSnapshot, Thread, ThreadId,
-};
-use indoc::indoc;
 use sqlez::{
     bindable::{Bind, Column},
     connection::Connection,
     statement::Statement,
 };
+use std::{
+    cell::{Ref, RefCell},
+    path::{Path, PathBuf},
+    rc::Rc,
+    sync::{Arc, Mutex},
+};
+use util::ResultExt as _;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum DataType {