Address Copilot review feedback:
- WriteTable now uses "Pushed:"/"Digest:" labels consistent with the
registry client's built-in output (pkg/registry/client.go:746-747),
so the default --output table experience is familiar to existing users
- Add TestPushOutputFlagCompletion to verify the --output flag is
properly registered and offers json/yaml/table completions
- Document that Pusher.Push and action.Push.Run signature changes are
intentional breaking changes in the Helm v4 major release
Signed-off-by: Ilya Kiselev <kis-ilya-a@yandex.ru>