fix(install): guard directory enumeration so -WhatIf no longer throws

CHANGELOG.md

@@ -27,6 +27,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
   [`README.md`](README.md) and [`AGENTS.md`](AGENTS.md) follow. The *Folder Structure* section now shows the deployed tree and the package layout side by side instead of conflating them, and the *Agent plugin* install path was rewritten: it previously told the reader that "the plugin format does not carry `.instructions.md` files or hooks, so this path gives you agents and skills only", which the migration makes false in the one direction that matters — hooks are exactly the guardrails a plugin-only user was silently missing. It now also names the two things worth knowing before choosing that path: bundled hooks execute locally and should be reviewed, and `rules`/`commands` registration is unconfirmed.
 
+### Fixed
+
+- **`Install-CopilotAtelier -WhatIf` no longer throws** (2026-08-27). A dry run aborted with a terminating `ItemNotFoundException` from `Get-ChildItem: Cannot find path '...\CopilotAtelier' because it does not exist`. The legacy-directory sweep enumerated the canonical target unconditionally, but under `-WhatIf` the target tree is never created (the `New-Item`/`Copy-Item` calls that build it honour `ShouldProcess` and are skipped), so the enumeration hit a path that did not exist. The sweep now runs only when the target is present, which matches how the rest of the function already guards optional paths with `Test-Path`. Covered by a regression test that asserts `-WhatIf` neither throws nor creates the canonical target.
+
 ## [4.0.0] - 2026-08-26
 
 ### Added

source/Public/Install-CopilotAtelier.ps1

@@ -281,9 +281,17 @@ function Install-CopilotAtelier
     #>
     $legacyDirectoryName = @('Agents', 'Instructions', 'Skills', 'Prompts', 'Hooks', 'Keybindings')
 
+    <#
+        The target tree only exists after the loop above created it. Under
+        -WhatIf that creation is skipped, so guard the enumeration against a
+        missing path instead of letting Get-ChildItem throw a terminating error.
+    #>
     $staleDirectory = @(
-        Get-ChildItem -LiteralPath $path.TargetPath -Directory |
-            Where-Object -FilterScript { $_.Name -cin $legacyDirectoryName }
+        if (Test-Path -LiteralPath $path.TargetPath)
+        {
+            Get-ChildItem -LiteralPath $path.TargetPath -Directory |
+                Where-Object -FilterScript { $_.Name -cin $legacyDirectoryName }
+        }
     )
 
     foreach ($directory in $staleDirectory)

tests/Unit/Public/Install-CopilotAtelier.Tests.ps1

@@ -262,4 +262,31 @@ Describe 'Install-CopilotAtelier' -Tag 'Unit' {
                 Should -Throw -ExpectedMessage "*No customization directory found*"
         }
     }
+
+    Context 'When previewing with -WhatIf' {
+        It 'Should not throw and should not create the canonical target' {
+            $whatIfContentPath = Join-Path -Path $TestDrive -ChildPath 'whatif-content'
+            $whatIfHomePath = Join-Path -Path $TestDrive -ChildPath 'whatif-home'
+            $whatIfConfigPath = Join-Path -Path $TestDrive -ChildPath 'whatif-config'
+
+            Initialize-CustomizationContent -Path $whatIfContentPath
+
+            $original = Enter-Sandbox -HomePath $whatIfHomePath -ConfigPath $whatIfConfigPath
+
+            try
+            {
+                { Install-CopilotAtelier -ContentPath $whatIfContentPath -SkipCopilotCliEnvironment -WhatIf } |
+                    Should -Not -Throw
+            }
+            finally
+            {
+                Exit-Sandbox -Original $original
+            }
+
+            # A dry run must not touch the file system.
+            $targetPath = Join-Path -Path $whatIfHomePath -ChildPath 'CopilotAtelier'
+
+            Test-Path -LiteralPath $targetPath | Should -BeFalse
+        }
+    }
 }