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/tool-registry.ts

@@ -572,9 +572,6 @@ export function setToolEnabled(toolName: string, enabled: boolean): void {
   else enabledTools.delete(toolName);
 }
 
-export function isToolEnabled(toolName: string): boolean {
-  return enabledTools.has(toolName);
-}
 
 export function getEnabledToolDefinitions(): ToolDefinition[] {
   const builtIn = TOOL_DEFINITIONS.filter(def => enabledTools.has(def.function.name));