fix: keep common.ps1 ASCII-only (PowerShell 5.1 compatibility)

My reworded Test-DirHasFiles comment introduced an em dash (U+2014), which tripped tests/test_ps1_encoding.py::test_ps1_file_is_ascii_only -- .ps1 files must stay ASCII for Windows PowerShell 5.1. Replace it with '--', matching the existing comment style in this file (e.g. the Resolve-SpecifyInitDir docstring).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jawwad-ali

scripts/powershell/common.ps1

@@ -210,7 +210,7 @@ function Test-FileExists {
 function Test-DirHasFiles {
     param([string]$Path, [string]$Description)
     # A directory counts as non-empty when Get-ChildItem returns any entry
-    # (files or subdirectories) — matching the JSON contracts checks in
+    # (files or subdirectories) -- matching the JSON contracts checks in
     # check-prerequisites.ps1 / setup-tasks.ps1, and treating a directory whose
     # only contents are subdirectories (e.g. contracts/v1/openapi.yaml) as
     # non-empty like bash check_dir. Filtering out subdirectories would