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

[sosterwinter]

Summary

Install-CopilotAtelier -WhatIf aborted with a terminating
ItemNotFoundException ("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
(New-Item/Copy-Item honour ShouldProcess and are skipped), so the
enumeration hit a path that did not exist.

Fix

Run the sweep only when the target exists, matching how the rest of the function
already guards optional paths with Test-Path.

Tests

Added a regression test asserting -WhatIf neither throws nor creates the
canonical target. Full Install-CopilotAtelier suite: 26 passed, 0 failed.

Fixes #23