feat: 增强聊天 UI 与终端配置

- 重构消息列表、思考块与工具调用块样式
- ChatContext 支持 agent 分段与 markdown 渲染优化
- 后端增加 PI 命令配置与 Web 终端改进
- 新增 run-backend.sh 启动脚本，忽略 backend/dist 构建产物

run-backend.bat

@@ -6,7 +6,7 @@ rem -------------------------------------------------------
 
 cd /d "%~dp0backend"
 echo Starting Go backend on port %BACKEND_PORT%...
-go run ./cmd/server --port %BACKEND_PORT% --agent-dir "%SPROUTCLAW_DIR%\.sproutclaw\agent" --pi-cmd "node %SPROUTCLAW_DIR%\packages\coding-agent\src\cli.ts"
+go run ./cmd/server --port %BACKEND_PORT% --agent-dir "%SPROUTCLAW_DIR%\.sproutclaw\agent" --repo-root "%SPROUTCLAW_DIR%" --pi-cmd "node %SPROUTCLAW_DIR%\packages\coding-agent\dist\cli.js"
 
 echo.
 pause >nul