chore: drop what the base-images refactor left behind

scripts/docker/slim.sh survived the removal of the stage that copied and ran
it, and nothing else referenced it.

The README still offered a `Virgin` image type with `pkgmgr-arch-virgin` as its
example, which is no longer published. Its second row was wrong independently
of that refactor: the image does not contain Nix, it runs `make install`, which
is scripts/installation/init.sh, and neither that nor entry.sh installs it. The
section now states the tags compute_publish_tags() actually pushes, including
the unsuffixed alias the default distribution gets, and points at base-images
for the clean environment the virgin image used to serve. A missing blank line
before a horizontal rule had also been rendering the paragraph above it as a
heading.

Three comment blocks explained what the repository used to do rather than what
it does, in the Dockerfile, in scripts/build/publish.sh and in the Makefile.
That belongs in the history, which is where it already is.

Verified: markdownlint-cli2 on the README goes from 21 findings to 10, all
remaining ones pre-existing and outside this section; `bash -n` on publish.sh
and `make -n build` pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/build/publish.sh

@@ -11,9 +11,6 @@ set -euo pipefail
 #   REGISTRY   (default: ghcr.io)
 #   IS_STABLE  (default: false)
 #   DISTROS    (default: "arch debian ubuntu fedora centos")
-#
-# The `virgin` variant moved to https://github.com/kevinveenbirkenbach/base-images
-# and `slim` is gone: one image per distribution, no target to pick.
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"