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/eval/Cargo.toml

@@ -19,6 +19,7 @@ path = "src/explorer.rs"
 
 [dependencies]
 agent.workspace = true
+agent_ui.workspace = true
 agent_settings.workspace = true
 anyhow.workspace = true
 assistant_tool.workspace = true

crates/eval/src/eval.rs

@@ -423,7 +423,7 @@ pub fn init(cx: &mut App) -> Arc<AgentAppState> {
     terminal_view::init(cx);
     let stdout_is_a_pty = false;
     let prompt_builder = PromptBuilder::load(fs.clone(), stdout_is_a_pty, cx);
-    agent::init(
+    agent_ui::init(
         fs.clone(),
         client.clone(),
         prompt_builder.clone(),

crates/eval/src/example.rs

@@ -100,7 +100,7 @@ impl ExampleContext {
     pub fn new(
         meta: ExampleMetadata,
         log_prefix: String,
-        agent_thread: Entity<agent::Thread>,
+        agent_thread: Entity<Thread>,
         model: Arc<dyn LanguageModel>,
         app: AsyncApp,
     ) -> Self {