fix: paths-only skips branch validation, setup-plan preserves existing plan (#2672)

* fix: paths-only skips branch validation, setup-plan preserves existing plan (#2653)

- check-prerequisites.sh/ps1: move branch validation after --paths-only
  early exit so --paths-only returns paths without requiring a spec branch
- setup-plan.sh/ps1: skip template copy when plan.md already exists to
  prevent overwriting user-authored plans on reruns
- setup-plan.sh: send status messages to stderr in --json mode so stdout
  remains parseable JSON
- Add tests for both fixes (bash + PowerShell)

* fix: remove trailing whitespace in PowerShell scripts

* fix: route PS skip message to stderr in -Json mode, add PS JSON assertions

Address review: setup-plan.ps1 Write-Output polluted stdout in -Json
mode when plan.md already existed. Use [Console]::Error.WriteLine()
when -Json is set. Add json.loads + stderr assertions to the PS rerun
test to catch regressions.

* fix: use Test-Path -PathType Leaf for plan existence check

Bare Test-Path matches directories too, which would silently skip plan
creation if a directory existed at the plan.md path.

scripts/powershell/check-prerequisites.ps1

@@ -56,14 +56,10 @@ EXAMPLES:
 # Source common functions
 . "$PSScriptRoot/common.ps1"
 
-# Get feature paths and validate branch
+# Get feature paths
 $paths = Get-FeaturePathsEnv
 
-if (-not (Test-FeatureBranch -Branch $paths.CURRENT_BRANCH -HasGit:$paths.HAS_GIT)) { 
-    exit 1 
-}
-
-# If paths-only mode, output paths and exit (support combined -Json -PathsOnly)
+# If paths-only mode, output paths and exit (no validation)
 if ($PathsOnly) {
     if ($Json) {
         [PSCustomObject]@{
@@ -85,6 +81,11 @@ if ($PathsOnly) {
     exit 0
 }
 
+# Validate branch name
+if (-not (Test-FeatureBranch -Branch $paths.CURRENT_BRANCH -HasGit:$paths.HAS_GIT)) {
+    exit 1
+}
+
 # Validate required directories and files
 if (-not (Test-Path $paths.FEATURE_DIR -PathType Container)) {
     Write-Output "ERROR: Feature directory not found: $($paths.FEATURE_DIR)"

scripts/powershell/setup-plan.ps1

@@ -33,17 +33,25 @@ if (-not (Test-FeatureJsonMatchesFeatureDir -RepoRoot $paths.REPO_ROOT -ActiveFe
 # Ensure the feature directory exists
 New-Item -ItemType Directory -Path $paths.FEATURE_DIR -Force | Out-Null
 
-# Copy plan template if it exists, otherwise note it or create empty file
-$template = Resolve-Template -TemplateName 'plan-template' -RepoRoot $paths.REPO_ROOT
-if ($template -and (Test-Path $template)) { 
-    # Read the template content and write it to the implementation plan file with UTF-8 encoding without BOM
-    $content = [System.IO.File]::ReadAllText($template)
-    $utf8NoBom = New-Object System.Text.UTF8Encoding($false)
-    [System.IO.File]::WriteAllText($paths.IMPL_PLAN, $content, $utf8NoBom)
+# Copy plan template if plan doesn't already exist
+if (Test-Path $paths.IMPL_PLAN -PathType Leaf) {
+    if ($Json) {
+        [Console]::Error.WriteLine("Plan already exists at $($paths.IMPL_PLAN), skipping template copy")
+    } else {
+        Write-Output "Plan already exists at $($paths.IMPL_PLAN), skipping template copy"
+    }
 } else {
-    Write-Warning "Plan template not found"
-    # Create a basic plan file if template doesn't exist
-    New-Item -ItemType File -Path $paths.IMPL_PLAN -Force | Out-Null
+    $template = Resolve-Template -TemplateName 'plan-template' -RepoRoot $paths.REPO_ROOT
+    if ($template -and (Test-Path $template)) {
+        # Read the template content and write it to the implementation plan file with UTF-8 encoding without BOM
+        $content = [System.IO.File]::ReadAllText($template)
+        $utf8NoBom = New-Object System.Text.UTF8Encoding($false)
+        [System.IO.File]::WriteAllText($paths.IMPL_PLAN, $content, $utf8NoBom)
+    } else {
+        Write-Warning "Plan template not found"
+        # Create a basic plan file if template doesn't exist
+        New-Item -ItemType File -Path $paths.IMPL_PLAN -Force | Out-Null
+    }
 }
 
 # Output results