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>

todos

@@ -0,0 +1,15 @@
+when developing, vm creation may fail, and firecracker logs need to be manually looked into, we should add a convenient way of digging through it. or perhaps log the last lines of it when vm creation fails
+
+`banger vm run` can hang waiting for ssh if things go south with sshd inside the vm. I think we should have a timeout instead of hanging forever. And also log what the user can do in such scenario.
+
+some commands are expected to take a while, it'd be good to at least show an indicator that banger is not hanging but rather doing something expected
+
+perhaps add an "interactive flag"?
+
+my computer is not the usual computer that users may have. perhaps it would be a good idea to screen the hardware we're working on so that we can set reasonable defaults for people when installing?
+
+versioning and releasing could use some love
+
+coverage would be somewhat nice to have
+
+regular users have no idea how to point their machine DNS to use the banger dns server. they need a tutorial/docs for this