fix: Add e2e build tag to Makefile test-e2e target

The test-e2e target was failing because it didn't include the -tags=e2e
build flag required by the E2E test files. All E2E test files have the
//go:build e2e directive, so the go test command needs -tags=e2e to
discover and run them.

Changes:
- Makefile:221: Added -tags=e2e to go test command

This enables running E2E tests with:
  make test-e2e

Related to Task #3765: E2E cluster readiness validation

Author: mattmattox

Makefile

@@ -218,7 +218,7 @@ test-integration:
 test-e2e:
 	@$(call print_status,"Running E2E tests...")
 	@if [ -d "test/e2e" ]; then \
-		go test ./test/e2e/... -v -timeout 10m; \
+		go test -tags=e2e ./test/e2e/... -v -timeout 10m; \
 	else \
 		$(call print_warning,"E2E tests not yet implemented (test/e2e/ does not exist)"); \
 	fi