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/context_server_tool.rs

@@ -4,9 +4,9 @@ use anyhow::{Result, anyhow, bail};
 use assistant_tool::{ActionLog, Tool, ToolResult, ToolSource};
 use context_server::{ContextServerId, types};
 use gpui::{AnyWindowHandle, App, Entity, Task};
+use icons::IconName;
 use language_model::{LanguageModel, LanguageModelRequest, LanguageModelToolSchemaFormat};
 use project::{Project, context_server_store::ContextServerStore};
-use ui::IconName;
 
 pub struct ContextServerTool {
     store: Entity<ContextServerStore>,