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/kernelcat/catalog.json

@@ -10,16 +10,6 @@
       "tarball_sha256": "d6f9ba2a957260063241cf9d79ae538d0c349107d37f0bfccc33281d29bd0901",
       "size_bytes": 9098722,
       "description": "Generic Firecracker kernel 6.12.8 (all drivers built-in, no initrd needed)"
-    },
-    {
-      "name": "void-6.12",
-      "distro": "void",
-      "arch": "x86_64",
-      "kernel_version": "6.12.81_1",
-      "tarball_url": "https://kernels.thaloco.com/void-6.12-x86_64.tar.zst",
-      "tarball_sha256": "3de6d03c4a3b5d3b8164f20049ddcb38b32a1864ea7133f01ff7fbb56c34d428",
-      "size_bytes": 187734807,
-      "description": "Void Linux 6.12 kernel for Firecracker microVMs"
     }
   ]
 }

internal/kernelcat/import.go

@@ -18,8 +18,9 @@ type DiscoveredArtifacts struct {
 	ModulesDir string
 }
 
-// metadataFile is the JSON dropped by scripts/make-void-kernel.sh alongside
-// its staged output. We read it when present to avoid guessing at filenames.
+// metadataFile is the optional JSON a kernel-build script can drop
+// alongside its staged output to point ReadLocal at specific filenames
+// without guessing.
 type metadataFile struct {
 	KernelPath string `json:"kernel_path"`
 	InitrdPath string `json:"initrd_path"`