Background

`observal sandbox` is missing a documentation page.

What needs to be done

Create `docs/cli/sandbox.md` documenting every subcommand. Use `observal sandbox --help` as the source of truth.

Files to touch

• `docs/cli/sandbox.md` (new)
• `docs/SUMMARY.md` — add link.
• `docs/cli/README.md` — add link.

Reference implementation

• `docs/cli/agent.md`

Acceptance criteria

• Every subcommand documented.
• Flags match the CLI output.
• Linked from `docs/SUMMARY.md` and `docs/cli/README.md`.