refactor: use official golangci-lint image for lint stage

Restructure Dockerfile to match upaas/dnswatcher pattern:
- Separate lint stage using golangci/golangci-lint:v2.1.6 image
- Builder stage for tests and compilation (no lint dependency)
- Add fmt-check Makefile target
- Decouple test from lint in Makefile (lint runs in its own stage)
- Run gofmt on all files
- docker build verified passing locally

internal/cli/integration_test.go

@@ -2285,6 +2285,7 @@ func verifyFileExists(t *testing.T, path string) {
 }
 
 // verifyFileNotExists checks if a file does not exist at the given path
+//
 //nolint:unused // kept for future use
 func verifyFileNotExists(t *testing.T, path string) {
 	t.Helper()