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_ui/src/ui.rs

@@ -0,0 +1,13 @@
+mod agent_notification;
+mod animated_label;
+mod context_pill;
+mod max_mode_tooltip;
+mod onboarding_modal;
+pub mod preview;
+mod upsell;
+
+pub use agent_notification::*;
+pub use animated_label::*;
+pub use context_pill::*;
+pub use max_mode_tooltip::*;
+pub use onboarding_modal::*;