Replace the old `void-exp` repository defaults with `void` so the Make targets,
registration helper, example config, verification messaging, and sample test
fixtures all line up with the new managed image name.
Keep the scope to repo-facing naming only: config overrides, helper output, and
test fixtures now expect `void`, while runtime compatibility for existing local
`void-exp` VMs remains an operational concern outside this commit.
Validation: go test ./..., make build, and a local `banger vm create --image void`
smoke boot with ssh and opencode ports up.
Thales Maciel
497e6dca3d