docs: add Windows NSIS download for Agent Arka 2026.9.1

Point anonymous users at Agent Arka_*_x64-setup.exe on the public release, and keep the Linux/WSL curl installer as the web-dashboard path.

agent-arka/install.sh

@@ -74,7 +74,7 @@ detect_platform() {
         linux) PLATFORM="linux-${arch}" ;;
         darwin) PLATFORM="darwin-${arch}" ;;
         mingw*|msys*|cygwin*)
-            die "On Windows use WSL, then this installer, or wait for the Desktop NSIS build. PowerShell: wsl --install"
+            die "This curl installer is for Linux/WSL. On Windows download Agent Arka_*_x64-setup.exe from https://git.arka-ai.ru/arka/public/releases/tag/agent-arka-v2026.9.1 (SmartScreen: More info → Run anyway). Or install WSL and re-run: wsl --install"
             ;;
         *) die "Unsupported OS: $os" ;;
     esac