Add /compress command to force a compression of the context (#986)

Related to https://b.corp.google.com/issues/423605555 - I figured this might be a simpler solution to start with, while still also being useful on its own even if we do implement that.

packages/cli/src/ui/components/HistoryItemDisplay.tsx

@@ -13,6 +13,7 @@ import { InfoMessage } from './messages/InfoMessage.js';
 import { ErrorMessage } from './messages/ErrorMessage.js';
 import { ToolGroupMessage } from './messages/ToolGroupMessage.js';
 import { GeminiMessageContent } from './messages/GeminiMessageContent.js';
+import { CompressionMessage } from './messages/CompressionMessage.js';
 import { Box } from 'ink';
 import { AboutBox } from './AboutBox.js';
 import { StatsDisplay } from './StatsDisplay.js';
@@ -81,5 +82,8 @@ export const HistoryItemDisplay: React.FC<HistoryItemDisplayProps> = ({
         isFocused={isFocused}
       />
     )}
+    {item.type === 'compression' && (
+      <CompressionMessage compression={item.compression} />
+    )}
   </Box>
 );