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/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 {