chore: add cross-platform startup scripts

- start.sh: Linux/macOS一键启动 (编译+后端+前端+健康检查)
- start.bat: Windows批处理 (两个终端窗口)
- Ctrl+C优雅退出，环境变量可覆盖

start.bat

@@ -0,0 +1,36 @@
+@echo off
+setlocal
+cd /d "%~dp0"
+
+set LLM_API_KEY=sk-test
+set JWT_SECRET=cyrene-v2-dev-secret-key-32bytes!!
+set ADMIN_PASSWORD=admin12345
+
+echo ============================================
+echo   Cyrene v2 — Windows 启动脚本
+echo ============================================
+
+echo [1/2] 编译后端...
+cd backend
+go build -o cyrene.exe ./cmd/cyrene/
+if %errorlevel% neq 0 (
+    echo 后端编译失败
+    pause
+    exit /b 1
+)
+
+echo [1/2] 启动后端 (http://localhost:8080)...
+start "Cyrene Backend" cmd /c "set LLM_API_KEY=%LLM_API_KEY% && set JWT_SECRET=%JWT_SECRET% && set ADMIN_PASSWORD=%ADMIN_PASSWORD% && cyrene.exe"
+
+cd ..\frontend\web
+
+echo [2/2] 启动前端 (http://localhost:5173)...
+start "Cyrene Frontend" cmd /c "npx vite --host 0.0.0.0"
+
+echo.
+echo Cyrene v2 启动完成
+echo   前端: http://localhost:5173
+echo   后端: http://localhost:8080
+echo   账号: admin / %ADMIN_PASSWORD%
+echo.
+pause