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/zed/src/main.rs

@@ -531,7 +531,7 @@ pub fn main() {
             cx,
         );
         let prompt_builder = PromptBuilder::load(app_state.fs.clone(), stdout_is_a_pty(), cx);
-        agent::init(
+        agent_ui::init(
             app_state.fs.clone(),
             app_state.client.clone(),
             prompt_builder.clone(),