Pin .NET 8 SDK to prevent CI formatter version mismatch

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: Prophet731

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "8.0.0",
+    "rollForward": "latestFeature"
+  }
+}