refactor: 全面清理死代码 — 删除 348 行无用代码

删除文件:
- training-export.ts: 训练数据导出（零引用，UI 已移除）

删除函数:
- workspace.ts: isProcessAlive, getActiveProcessCount, execWorkspaceCommand, terminateWorkspaceCommand
- sqlite.ts: deleteMessagesBySession, getSkill, closeDatabase
- tool-registry.ts: isToolEnabled
- cron-manager.ts: clearAllTimers
- workspace-panel.ts: setOnToolTerminated, setWorkspaceDirPath

删除 UI:
- index.html: 训练数据导出按钮及说明
- settings-modal.ts: 训练数据导出事件绑定

src/renderer/services/cron-manager.ts

@@ -183,12 +183,3 @@ export async function restoreCronTasks(): Promise<void> {
     logInfo(`定时任务已恢复: ${scheduled} 个任务`);
   }
 }
-
-/**
- * 清理所有定时器（应用退出时调用）
- */
-export function clearAllTimers(): void {
-  for (const [id] of activeTimers) {
-    clearTimer(id);
-  }
-}