fix(ps1): replace null-conditional operator for PowerShell 5.1 compatibility (#1975)

The `?.` (null-conditional member access) operator requires PowerShell 7.1+,
but Windows ships with PowerShell 5.1 by default. When AI agents invoke .ps1
scripts on Windows, they typically use the system-associated handler (5.1),
causing a ParseException: Unexpected token '?.Path'.

Replace the single `?.` usage with a 5.1-compatible two-step pattern that
preserves the same null-safety behavior.

Fixes #1972

scripts/powershell/common.ps1

@@ -8,7 +8,8 @@ function Find-SpecifyRoot {
 
     # Normalize to absolute path to prevent issues with relative paths
     # Use -LiteralPath to handle paths with wildcard characters ([, ], *, ?)
-    $current = (Resolve-Path -LiteralPath $StartDir -ErrorAction SilentlyContinue)?.Path
+    $resolved = Resolve-Path -LiteralPath $StartDir -ErrorAction SilentlyContinue
+    $current = if ($resolved) { $resolved.Path } else { $null }
     if (-not $current) { return $null }
 
     while ($true) {