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
Change NewCLIInstance() and NewCLIInstanceWithFs() to return
(*Instance, error) instead of panicking on DetermineStateDir failure.
Callers in RunE contexts propagate the error. Callers in command
construction (for shell completion) use log.Fatalf. Test callers
use t.Fatalf.
Addresses review feedback on PR #18.