cmd/atlas: add tests for startup and update-check behaviour

While setting up the project locally, I noticed that cmd/atlas/main.go contains several user-facing startup behaviors that currently do not have focused unit tests.

In particular, tests would be useful to verify that:

ATLAS_NO_UPDATE_NOTIFIER disables update checks. Development builds with invalid or unset versions skip update checks. Adding these targeted tests would help prevent regressions while keeping the existing implementation unchanged.