Add ImageMagick and Ghostscript (closes #7)

Install imagemagick (ImageMagick 7, so the CLI is magick) and
ghostscript from apt and check both in the smoke test.

Model: deepseek-v4-pro

README.md

@@ -39,6 +39,8 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | sqlite3 (CLI)   | apt                            | `/usr/bin/sqlite3`             |
 | libvips         | apt `libvips-dev`, `libvips-tools` | `vips` on `PATH`          |
 | libheif         | apt `libheif-dev`, `libheif-plugin-aomenc` | HEIF/AVIF codec   |
+| ImageMagick     | apt                            | `magick`                       |
+| Ghostscript     | apt                            | `gs`                           |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`