fix: 优化 ModelPicker 副标题和 resume 错误提示的可操作性

- ModelPicker 副标题从技术说明改为操作提示（effort 调整、1M 切换）
- /resume 错误提示添加 "Run /resume to browse" 操作引导
- 新增 6 个测试覆盖模型选择器、会话恢复和 cost 消息文案

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/ModelPicker.tsx

@@ -229,7 +229,7 @@ export function ModelPicker({
           </Text>
           <Text dimColor>
             {headerText ??
-              'Switch between Claude models. Applies to this session and future Claude Code sessions. For other/previous model names, specify with --model.'}
+              'Choose a model for this and future sessions. Use ← → to adjust effort, Space to toggle 1M context.'}
           </Text>
           {sessionModel && (
             <Text dimColor>