feat: add /compact (#289)

Added the ability to compact. Not sure if I should switch the model over
to gpt-4.1 for longer context or if keeping the current model is fine.
Also I'm not sure if setting the compacted to system is best practice,
would love feedback 😄

Mentioned in this issue: https://github.com/openai/codex/issues/230

codex-cli/src/components/help-overlay.tsx

@@ -52,6 +52,9 @@ export default function HelpOverlay({
         <Text>
           <Text color="cyan">/clearhistory</Text> – clear command history
         </Text>
+        <Text>
+          <Text color="cyan">/compact</Text> – condense context into a summary
+        </Text>
 
         <Box marginTop={1}>
           <Text bold dimColor>