fix: resolve all golangci-lint errors and integrate linter into CI/CD, Makefile, and check script

scripts/check.sh

@@ -15,9 +15,14 @@ if [ -n "$UNFORMATTED" ]; then
 fi
 echo "✓ Go files formatted correctly."
 
-echo "==> 2. Running Go vet..."
+echo "==> 2. Running Go vet & golangci-lint..."
 go vet ./...
-echo "✓ Go vet passed."
+if command -v golangci-lint &> /dev/null; then
+    golangci-lint run
+    echo "✓ golangci-lint passed (0 issues)."
+else
+    echo "⚠ golangci-lint not installed locally (will run in CI)."
+fi
 
 echo "==> 3. Running Go tests..."
 go test -v ./...