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

Makefile

@@ -42,12 +42,16 @@ check:
 	@chmod +x scripts/check.sh
 	@./scripts/check.sh
 
-## lint: Run gofmt and go vet
+## lint: Run gofmt, go vet, and golangci-lint
 lint:
 	@echo "==> Verifying Go formatting..."
 	@test -z "$$(gofmt -l internal/ cmd/)" || (echo "Unformatted files:" && gofmt -l internal/ cmd/ && exit 1)
 	@echo "==> Running Go vet..."
 	@go vet ./...
+	@if command -v golangci-lint &> /dev/null; then \
+		echo "==> Running golangci-lint..."; \
+		golangci-lint run; \
+	fi
 	@echo "✓ Lint passed."
 
 ## format: Format all Go files with gofmt