fix: write prerequisite error messages to stderr in setup-tasks.ps1

Author: Nimraakram12

scripts/powershell/setup-tasks.ps1

@@ -25,14 +25,14 @@ if (-not (Test-FeatureBranch -Branch $paths.CURRENT_BRANCH -HasGit:$paths.HAS_GI
 
 # Validate prerequisites
 if (-not (Test-Path $paths.FEATURE_DIR -PathType Container)) {
-    Write-Output "ERROR: Feature directory not found: $($paths.FEATURE_DIR)"
-    Write-Output "Run /speckit.specify first to create the feature structure."
+    [Console]::Error.WriteLine("ERROR: Feature directory not found: $($paths.FEATURE_DIR)")
+    [Console]::Error.WriteLine("Run /speckit.specify first to create the feature structure.")
     exit 1
 }
 
 if (-not (Test-Path $paths.IMPL_PLAN -PathType Leaf)) {
-    Write-Output "ERROR: plan.md not found in $($paths.FEATURE_DIR)"
-    Write-Output "Run /speckit.plan first to create the implementation plan."
+    [Console]::Error.WriteLine("ERROR: plan.md not found in $($paths.FEATURE_DIR)")
+    [Console]::Error.WriteLine("Run /speckit.plan first to create the implementation plan.")
     exit 1
 }