Initial WindowGrid KWin effect for Plasma 6

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

metadata.json

@@ -0,0 +1,16 @@
+{
+    "KPlugin": {
+        "Authors": [
+            {
+                "Name": "Chris",
+                "Email": "work@cdnutter.org"
+            }
+        ],
+        "Category": "Window Management",
+        "Description": "Drag a window, hold the right mouse button or Space to pick grid cells, and the window snaps to them",
+        "EnabledByDefault": false,
+        "Id": "windowgrid",
+        "License": "GPL-3.0-or-later",
+        "Name": "WindowGrid"
+    }
+}