Scaffold a project sandbox

This installs a self-contained Docker sandbox in the current project directory, so all builds/runs/tests happen inside a container instead of on the host.

Steps

1. Determine the sandbox name.
   • Base name: $ARGUMENTS if passed, otherwise $(basename "$PWD").
   • Append a 6-char hash of the absolute project path, so two unrelated projects that happen to share a base name never collide:

     HASH=$(echo -n "$(realpath .)" | sha256sum | cut -c1-6)
     NAME="<base-name>-${HASH}"
     

   • This resolved NAME is what __NAME__ gets substituted with everywhere below. It's deterministic (same path → same name every time), so re-running this skill or sandbox.sh rebuild from the same directory always targets the same container — never generate a random suffix.