codex-rs: Rename /clear to /new, make it start an entirely new chat (#1264)

I noticed that `/clear` wasn't fully clearing chat history; it would
clear the chat history widgets _in the UI_, but the LLM still had access
to information from previous messages.

This PR renames `/clear` to `/new` for clarity as per Michael's
suggestion, resetting `app_state` to a fresh `ChatWidget`.

codex-rs/tui/src/chatwidget.rs

@@ -207,11 +207,6 @@ impl ChatWidget<'_> {
         self.conversation_history.scroll_to_bottom();
     }
 
-    pub(crate) fn clear_conversation_history(&mut self) {
-        self.conversation_history.clear();
-        self.request_redraw();
-    }
-
     pub(crate) fn handle_codex_event(&mut self, event: Event) {
         let Event { id, msg } = event;
         match msg {