Add /compact (#1527)

- Add operation to summarize the context so far.
- The operation runs a compact task that summarizes the context.
- The operation clear the previous context to free the context window
- The operation didn't use `run_task` to avoid corrupting the session
- Add /compact in the tui



https://github.com/user-attachments/assets/e06c24e5-dcfb-4806-934a-564d425a919c

codex-rs/tui/src/chatwidget.rs

@@ -502,6 +502,12 @@ impl ChatWidget<'_> {
     pub(crate) fn token_usage(&self) -> &TokenUsage {
         &self.token_usage
     }
+
+    pub(crate) fn clear_token_usage(&mut self) {
+        self.token_usage = TokenUsage::default();
+        self.bottom_pane
+            .set_token_usage(self.token_usage.clone(), self.config.model_context_window);
+    }
 }
 
 impl WidgetRef for &ChatWidget<'_> {