[Task]: Research & design the acceptance test harness

[michael-duren]

What needs to be done

Decide and document how we drive box end-to-end in acceptance tests, so the
implementation tasks have a settled design to build against. The output is a
short design note in the repo (e.g. docs/acceptance-testing.md) plus answers to
the epic's open questions.

Cover:

• Privilege model. Determine whether the suite targets root (sudo) or
  rootless (user namespaces + newuidmap/subuid). The current sample
  config.json has no user namespace, so document what's required today and what
  rootless would need.
• How to invoke the runtime. We drive the real CLI: box create <id> --bundle <dir>, box start <id>, box state <id>, box kill <id>, box delete <id>. Decide whether tests shell out to the built ./bin/box (build it
  first) or call internal/operations directly. Prefer driving the binary so the
  test mirrors real usage.
• Test framework. Go acceptance tests behind a //go:build acceptance tag
  vs. a bats/shell suite. Note how state directories ($XDG_STATE_HOME, etc.)
  are isolated per test so runs don't collide or touch a developer's real state.
• What to assert. OCI status transitions via box state (creating →
  created → running → stopped), exit codes, the user process actually
  running, and that delete removes the state + runtime dirs.
• Skip strategy. How the suite detects missing prerequisites (not Linux, no
  privileges, no rootfs) and skips with a clear reason instead of failing.

Acceptance criteria

• A design note committed summarizing harness shape, privilege model, and isolation
• Decision recorded: root vs. rootless for the first iteration, with rationale
• Decision recorded: Go (//go:build acceptance) vs. bats, with rationale
• A documented way to isolate runtime/state dirs per test run
• Follow-up tasks ([Task]: Build a reusable test bundle + rootfs fixture #19, [Task]: Implement the core create→start→kill→delete lifecycle test #20) refined based on findings

Parent / tracking issue

Part of #17.

Rough size

M — a day or two