Explain the sandbox security procedure in a deck for non-experts #905

Goal

Give testers, admins and newcomers a short slide deck that makes the platform's security procedure clear from an empty machine to steady-state operation, using the sandbox lane as the example.

Scope

• A picture story with one illustrated scene per step, followed by a text walkthrough. Each step shows what the admin does, what they see, and what the system holds afterwards.
• The centrepiece is approval with a phone, laptop or hardware key for temporary root tokens and restores, with the 30-day check-in.
• The deck describes the design. Every claim cites its source in sources.md, and points the design doesn't settle yet are left out rather than filled in.
• Published as a PDF with its editable sources in docs/presentations/security-and-bootstrap/.

Done when

• The deck, its sources and the build files are in the umbrella repo.
• Every claim on the slides traces to a source listed in sources.md.