feat: add support for default conversation via --conv default (#580)

Co-authored-by: Letta <noreply@letta.com>

src/cli/App.tsx

@@ -1729,6 +1729,7 @@ export default function App({
             stream = await sendMessageStream(
               conversationIdRef.current,
               currentInput,
+              { agentId: agentIdRef.current },
             );
           } catch (preStreamError) {
             // Check if this is a pre-stream approval desync error