Install opencode in Void rootfs

Bring the experimental Void image closer to the default dev image path by installing pinned mise inside the rootfs build, using it to install opencode, and activating mise automatically for root bash sessions.

Keep the change scoped to the Void builder rather than packages.void so the image still stays language-agnostic at the package-manifest level, then clean mise download/cache artifacts before sealing the rootfs and work-seed.

Extend verify-void so the smoke path now proves mise and opencode are actually present in a fresh void-exp VM. Verified with bash -n make-rootfs-void.sh verify.sh, GOCACHE=/tmp/banger-gocache go test ./..., and make build.

README.md

@@ -325,6 +325,8 @@ This path is intentionally local-only and does not change the default Debian
 image flow. It reuses the current runtime bundle kernel, initrd, and modules,
 but builds a lean `x86_64-glibc` Void userspace with:
 - `bash` installed for interactive/admin use
+- pinned `mise` installed at `/usr/local/bin/mise`, activated for `root` bash shells
+- `opencode` installed through `mise`, with `/usr/local/bin/opencode` available by default
 - `docker` plus `docker-compose` installed from Void packages
 - the `docker` runit service enabled, with Docker netfilter/forwarding kernel prep
 - `openssh` enabled under runit
@@ -332,9 +334,7 @@ but builds a lean `x86_64-glibc` Void userspace with:
 - `root` normalized to `/bin/bash` while keeping `/bin/sh` as the distro's system shell
 - a generated `/root` work-seed for fast creates
 
-It does not install the Debian-oriented extras from rebuilt default images:
-- no `mise`
-- no `opencode`
+It still keeps some Debian-oriented extras out for now:
 - no tmux plugin defaults
 
 The builder fetches official static XBPS tools and packages from the Void