fix: stop check-prerequisites --paths-only from writing feature.json (#3025) (#3190)

* fix: stop check-prerequisites --paths-only from writing feature.json (#3025)

check-prerequisites --paths-only / -PathsOnly is documented as pure,
read-only path resolution, but when SPECIFY_FEATURE_DIRECTORY was set it
called the persist routine and rewrote .specify/feature.json. That dirtied
the working tree and overwrote a pinned feature directory during what should
be a no-op.

Add an explicit opt-out at the resolver boundary instead of a global env
back-channel:

- bash: get_feature_paths accepts a leading --no-persist flag that skips
  _persist_feature_json; check-prerequisites.sh passes it in --paths-only mode.
- PowerShell: Get-FeaturePathsEnv gains a -NoPersist switch that skips
  Save-FeatureJson; check-prerequisites.ps1 passes it in -PathsOnly mode.

Normal (non-paths-only) invocations are unchanged and still persist the
override, so future sessions without the env var keep working.

Add regression tests asserting --paths-only/-PathsOnly leaves a pinned
feature.json untouched even when the env override differs, plus a guard that
normal mode still persists.

* fix: use ASCII hyphen in common.ps1 comment for PS 5.1 compatibility

The em-dash in the persist comment introduced non-ASCII bytes, failing
test_ps1_file_is_ascii_only which enforces ASCII-only PowerShell sources
for Windows PowerShell 5.1 compatibility.

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

* test: add PowerShell normal-mode persistence guard (#3025)

Addresses Copilot review feedback on #3190: the bash side had a
`test_normal_mode_still_persists_feature_json` guard, but there was no
symmetric PowerShell test asserting that running check-prerequisites.ps1
*without* -PathsOnly still persists the SPECIFY_FEATURE_DIRECTORY override
into .specify/feature.json.

Add test_ps_normal_mode_still_persists_feature_json, which guards against
accidentally passing -NoPersist unconditionally (or flipping the default)
in a future refactor. Verified it fails when -NoPersist is passed in the
non -PathsOnly branch and passes with the current conditional.

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

---------

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

scripts/powershell/common.ps1

@@ -143,6 +143,13 @@ function Save-FeatureJson {
 }
 
 function Get-FeaturePathsEnv {
+    # Read-only callers (e.g. check-prerequisites.ps1 -PathsOnly) pass -NoPersist
+    # so pure path resolution never writes .specify/feature.json, which would
+    # dirty the working tree or overwrite a pinned value (issue #3025).
+    param(
+        [switch]$NoPersist
+    )
+
     $repoRoot = Get-RepoRoot
     $currentBranch = Get-CurrentBranch
 
@@ -157,8 +164,11 @@ function Get-FeaturePathsEnv {
         if (-not [System.IO.Path]::IsPathRooted($featureDir)) {
             $featureDir = Join-Path $repoRoot $featureDir
         }
-        # Persist to feature.json so future sessions without the env var still work
-        Save-FeatureJson -RepoRoot $repoRoot -FeatureDirectory $env:SPECIFY_FEATURE_DIRECTORY
+        # Persist to feature.json so future sessions without the env var still
+        # work - unless the caller opted out for read-only resolution (#3025).
+        if (-not $NoPersist) {
+            Save-FeatureJson -RepoRoot $repoRoot -FeatureDirectory $env:SPECIFY_FEATURE_DIRECTORY
+        }
     } elseif (Test-Path $featureJson) {
         $featureJsonRaw = Get-Content -LiteralPath $featureJson -Raw
         try {