v1.16 — Remove PowerShell edition, restore flat project structure

PowerShell version removed — will use OV code signing certificate instead.
Files moved back from src/ to project root.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Models/OfficeVersion.cs

@@ -0,0 +1,9 @@
+namespace InstaSoftOfficeTool.Models
+{
+    public enum OfficeVersion
+    {
+        Office2024,
+        Office2021,
+        Office2019
+    }
+}