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

bin/test

@@ -27,7 +27,9 @@ docker run --rm "$img" bash -ec '
     zstd --version
     sqlite3 --version
     vips --version
-    vips -l | grep -i heif > /dev/null
+vips -l | grep -i heif > /dev/null
+    magick --version
+    gs --version
 '
 
 echo "== $img: runit service lifecycle"