Prune legacy void/alpine + customize.sh flows

The golden-image Dockerfile + catalog pipeline replaces the entire
manual rootfs-build stack. With that shipped, the per-distro shell
flows are dead code.

Removed:
- scripts/customize.sh, scripts/interactive.sh, scripts/verify.sh
- scripts/make-rootfs{,-void,-alpine}.sh
- scripts/register-{void,alpine}-image.sh
- scripts/make-{void,alpine}-kernel.sh
- internal/imagepreset/ (only consumer was `banger internal packages`,
  which fed customize.sh)
- examples/{void,alpine}.config.toml
- Makefile targets: rootfs, rootfs-void, rootfs-alpine, void-kernel,
  alpine-kernel, void-register, alpine-register, void-vm, alpine-vm,
  verify-void, verify-alpine, plus the ALPINE_RELEASE / *_IMAGE_NAME
  / *_VM_NAME variables

The void-6.12 kernel catalog entry is also gone — golden image pairs
with generic-6.12 and nothing else in the catalog depended on it.

Consolidated: imagemgr now holds the small DebianBasePackages list +
package-hash helper inline, so the `image build --from-image` flow
(still supported) no longer pulls from a separate imagepreset package.

Net: 3,815 lines deleted, 59 added. No runtime functionality removed
beyond the `banger internal packages` subcommand (hidden, used only
by the deleted customize.sh).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/cli_test.go

@@ -190,24 +190,6 @@ func TestInternalNATFlagsExist(t *testing.T) {
 	}
 }
 
-func TestInternalPackagesCommandSupportsAlpine(t *testing.T) {
-	cmd := NewBangerCommand()
-	var stdout bytes.Buffer
-	cmd.SetOut(&stdout)
-	cmd.SetArgs([]string{"internal", "packages", "alpine"})
-
-	if err := cmd.Execute(); err != nil {
-		t.Fatalf("Execute(): %v", err)
-	}
-
-	output := stdout.String()
-	for _, want := range []string{"alpine-base", "docker", "libgcc", "libstdc++", "mkinitfs", "openssh"} {
-		if !strings.Contains(output, want+"\n") {
-			t.Fatalf("output = %q, want package %q", output, want)
-		}
-	}
-}
-
 func TestPSAndVMListAliasesAndFlagsExist(t *testing.T) {
 	root := NewBangerCommand()
 	ps, _, err := root.Find([]string{"ps"})