feat(cli): add /btw slash command for ephemeral side questions

Allow users to ask quick "by the way" questions that use the current
conversation context but don't pollute the main conversation history.

Closes #2370

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/ui/components/messages/BtwMessage.tsx

@@ -0,0 +1,44 @@
+/**
+ * @license
+ * Copyright 2025 Qwen Code
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type React from 'react';
+import { Box, Text } from 'ink';
+import type { BtwProps } from '../../types.js';
+import Spinner from 'ink-spinner';
+import { Colors } from '../../colors.js';
+
+export interface BtwDisplayProps {
+  btw: BtwProps;
+}
+
+/**
+ * BtwMessage renders the /btw (by the way) sidebar response.
+ * Shows an ephemeral question and answer that doesn't affect the main conversation.
+ */
+export const BtwMessage: React.FC<BtwDisplayProps> = ({ btw }) => (
+    <Box flexDirection="column">
+      <Box flexDirection="row">
+        <Text color={Colors.Gray} dimColor>
+          {'btw> '}
+        </Text>
+        <Text color={Colors.Gray}>{btw.question}</Text>
+      </Box>
+      <Box flexDirection="row" marginTop={0}>
+        {btw.isPending ? (
+          <Box>
+            <Box marginRight={1}>
+              <Spinner type="dots" />
+            </Box>
+            <Text color={Colors.AccentPurple}>Thinking...</Text>
+          </Box>
+        ) : (
+          <Box flexDirection="column">
+            <Text color={Colors.AccentCyan}>{btw.answer}</Text>
+          </Box>
+        )}
+      </Box>
+    </Box>
+  );