[albertompe]

Summary

Make Robin report which build is running.

The image build already passed -ldflags "-X main.version=${VERSION}", but main.version was never declared, so the linker silently ignored it. This PR declares var version = "dev" in cmd/main.go and exposes the version:

• redkey_robin_build_info gauge (value 1, labels version and go_version), mirroring the operator's redkey_operator_build_info. It is registered on controller-runtime's registry rather than the resettable one, because ResettableRegistry.Reset() would drop it.
• --version flag: prints the version and exits.
• The version is included in the startup log.
• Kubernetes client User-Agent: redkey-robin/<version>.
• Makefile: make build and make run inject VERSION through LDFLAGS. docker-build and docker-buildx already passed it to the Dockerfile. A plain go build falls back to dev.

The Redis CLIENT SETNAME part of the issue is deferred, because CLIENT SETNAME can fail for ACL-restricted users.

No reconciliation behavior changes.

Closes #45

Additional context

• New tests:
  • internal/metrics/buildinfo_test.go checks the gauge value and labels.
  • cmd/main_test.go builds the binary with and without -X main.version and checks the --version output. It is skipped under -short.
• make lint and make test pass.
• make test-integration
• Manual check: make build && bin/robin --version prints 0.2.0; go build ./cmd/ prints dev.
• The dashboard panel and the docs are in the operator PR: InditexTech/redkey-operator#.