fix(tui): reset agent state on /reset command in fullscreen mode

Use agent.reset() instead of session.clear() when agent is available,
ensuring tool fingerprint, usage counters, and agent history are all
cleared properly alongside the session.

src/frontends/tui/components/App.tsx

@@ -149,7 +149,11 @@ export function App({
         return;
 
       case 'reset':
-        session.clear();
+        if (agent) {
+          agent.reset();
+        } else {
+          session.clear();
+        }
         setMessages([]);
         setTokenUsage({ inputTokens: 0, outputTokens: 0 });
         setScrollOffset(0);