use ratatui::crossterm re-exports, add event-stream feature

All crossterm imports go through ratatui::crossterm. Direct crossterm
dep kept only for the event-stream feature flag (EventStream for
async terminal input).

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/event_loop.rs

@@ -5,7 +5,7 @@
 // (agent, active tools) directly for rendering.
 
 use anyhow::Result;
-use crossterm::event::{Event, EventStream, KeyEventKind};
+use ratatui::crossterm::event::{Event, EventStream, KeyEventKind};
 use futures::StreamExt;
 use std::sync::Arc;
 use std::time::Duration;