fix(skills): guard pruneSkillBackups against non-DWS directories

Restrict backup pruning to directories whose names match the DWS stamp
format (YYYYmmdd-HHMMSS with optional -N suffix) across all 8 installer
surfaces (Go, npm, 4 shell, 2 PowerShell). Unknown directories in
~/.dws/skill-backups are now preserved. Also fixes Windows coverage test
portability and covers the remaining macOS changed-code gap (retire
warning loop in runUpgrade).

scripts/install-devapp.ps1

@@ -290,7 +290,11 @@ $script:DevSkillBackupRootsThisRun = [System.Collections.Generic.HashSet[string]
 function Remove-OldDevSkillBackups {
     $root = Join-Path $HOME ".dws\skill-backups"
     if (!(Test-Path -LiteralPath $root -PathType Container)) { return }
+    # Only directories whose names match the DWS backup stamp format (UTC
+    # yyyyMMdd-HHmmss, optional -N collision suffix) are candidates; any
+    # other entry is foreign data and is preserved.
     $dirs = @(Get-ChildItem -LiteralPath $root -Directory -Force -ErrorAction SilentlyContinue |
+        Where-Object { $_.Name -match '^[0-9]{8}-[0-9]{6}(-[0-9]+)?$' } |
         Sort-Object -Property Name)
     $excess = $dirs.Count - $DevSkillBackupKeep
     foreach ($dir in $dirs) {