Add interface design references and principles; implement sport service tests

- Introduced critique, example, principles, and validation documents for interface design.
- Enhanced backend service with coverage threshold and fixed variable declaration in sport service.
- Added comprehensive tests for create, update, and list sports functionalities.

skills-lock.json

@@ -0,0 +1,10 @@
+{
+  "version": 1,
+  "skills": {
+    "interface-design": {
+      "source": "dammyjay93/interface-design",
+      "sourceType": "github",
+      "computedHash": "d004c5e5e2e7132e1d64e9c2a339db7f7aaa69053a5c19cddc210dd35dd6fff5"
+    }
+  }
+}