fix: 后台下载改用 Start-Process，避免子进程继承管道句柄阻塞 hook

start /b 派生的进程会继承 Claude 传给 hook 的 stdout 管道句柄，curl 攥着不放
导致 Claude 读不到 EOF、直到下载完才放行（hook 被卡住）。改用 powershell
Start-Process 启动下载，它默认不继承父进程句柄，hook 立即返回、下载后台继续。

scripts/notify.cmd

@@ -29,7 +29,12 @@ if exist "%LOCK%" call :reclaim
 
 rem atomic lock: only the first hook spawns the worker; others fall through and just
 rem report progress -> no duplicate downloads even when hooks fire concurrently.
-mkdir "%LOCK%" 2>nul && start "" /b "%~f0" __download <nul >nul 2>&1
+rem
+rem must use Start-Process (not `start /b`): a child started by cmd inherits the
+rem hook's stdout pipe handle, so Claude won't see EOF until the download ends ->
+rem the hook would block. Start-Process spawns WITHOUT inheriting handles, so the
+rem hook returns immediately while curl keeps running detached.
+mkdir "%LOCK%" 2>nul && powershell -nop -w hidden -c "Start-Process -WindowStyle Hidden -FilePath '%~f0' -ArgumentList '__download'" >nul 2>&1
 
 rem downloaded size so far (from the .partial file), shown as X.X MB
 set "DLBYTES=0"