Rename experimental Void image to void

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.

internal/daemon/daemon_test.go

@@ -90,7 +90,7 @@ func TestPromoteImageCopiesBootArtifactsIntoArtifactDir(t *testing.T) {
 	db := openDaemonStore(t)
 	image := model.Image{
 		ID:         "img-promote",
-		Name:       "void-exp",
+		Name:       "void",
 		Managed:    false,
 		RootfsPath: rootfs,
 		KernelPath: kernel,