fix(portable/setup.ps1): cd to core dir before npm install

When users run setup.ps1 from PowerShell with cwd outside the repo
(e.g. C:\Users\Admin>), npm 11 walks up looking for package.json and
errors with ENOENT before honoring --prefix. setup.bat already pushd's
into core; setup.ps1 was missing the equivalent Push-Location guard.

Fixes #27 (setup.ps1 ENOENT package.json reported by lawler-code).

portable/setup.ps1

@@ -224,7 +224,13 @@ if (Test-Path -Path (Join-Path $coreDir "node_modules\openclaw") -PathType Conta
 }
 else {
     Write-Step "->" "Installing OpenClaw..." "Cyan"
+    Push-Location $coreDir
+    try {
         & $npmCmd install --prefix $coreDir --registry=$mirror
+    }
+    finally {
+        Pop-Location
+    }
 
     if (Test-Path -Path (Join-Path $coreDir "node_modules\openclaw") -PathType Container) {
         Write-Step "OK" "OpenClaw installation completed." "Green"
@@ -240,11 +246,15 @@ if (Test-Path -Path (Join-Path $coreDir "node_modules\@sliverp\qqbot") -PathType
 }
 else {
     Write-Step "->" "Installing QQ plugin..." "Cyan"
+    Push-Location $coreDir
     try {
-        & $npmCmd install @sliverp/qqbot@latest --prefix $coreDir --registry=$mirror 2>$null
+        & $npmCmd install "@sliverp/qqbot@latest" --prefix $coreDir --registry=$mirror 2>$null
     }
     catch {
     }
+    finally {
+        Pop-Location
+    }
     Write-Step "OK" "QQ plugin installation finished." "Green"
 }