Harden Remove-OldPSNowManifest: validate source template exists before deleting existing manifest

[johnmccrae]

Problem

Remove-OldPSNowManifest deletes the existing PlasterManifest.xml before checking whether the replacement source file ("$BaseManifest.xml") exists. If the source is missing, Copy-Item throws a generic error after the original manifest is already gone, leaving the template root in a broken state.

Files

• Private/Remove-OldPSNowManifest.ps1 (lines 14–27)

Acceptance Criteria

• Function throws a clear, terminating ErrorRecord if "$BaseManifest.xml" does not exist in PlasterTemplate/
• The throw happens before any Remove-Item calls — existing manifest is never deleted if replacement is missing
• New Pester tests cover the missing-template path and verify no Remove-Item is called
• Existing tests remain green