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/commands/resume/resume.tsx

@@ -36,9 +36,9 @@ type ResumeResult =
 function resumeHelpMessage(result: ResumeResult): string {
   switch (result.resultType) {
     case 'sessionNotFound':
-      return `Session ${chalk.bold(result.arg)} was not found.`;
+      return `Session ${chalk.bold(result.arg)} was not found. Run ${chalk.bold('/resume')} without arguments to browse all sessions.`;
     case 'multipleMatches':
-      return `Found ${result.count} sessions matching ${chalk.bold(result.arg)}. Please use /resume to pick a specific session.`;
+      return `Found ${result.count} sessions matching ${chalk.bold(result.arg)}. Run ${chalk.bold('/resume')} to pick one from the list.`;
   }
 }