[Task]: Add a 'make acceptance' target

[michael-duren]

What needs to be done

Add a make acceptance (and/or scripts/acceptance.sh) target that builds
box and runs the acceptance suite end-to-end with one command, so contributors
don't have to remember the build tag / privilege incantation.

The target should:

• Build the box binary first (reuse the existing make build → ./bin/box).
• Run the acceptance suite (e.g. go test -tags acceptance ./test/acceptance/...
  or the chosen runner from [Task]: Research & design the acceptance test harness #18).
• Handle the privilege model decided in the research task (e.g. re-exec under
  sudo, or document that the caller runs it privileged) and point state/runtime
  dirs at a throwaway location so a developer's real containers are untouched.
• Allow running a single test / subset for focused debugging.
• Skip (not fail) with a clear message when prerequisites are missing.

Acceptance criteria

• make acceptance builds box and runs the suite in one command
• Runtime/state dirs are isolated to a throwaway location for the run
• A documented way to run a single test or subset (e.g. make acceptance RUN=...)
• The target is documented in the README "Development" section
• Running it on a suitable Linux host produces a real pass/fail result

Parent / tracking issue

Part of #17.

Blocked by

• [Task]: Implement the core create→start→kill→delete lifecycle test #20

Rough size

S — a few hours