docs(demo): the wipe prunes the images its pull replaced

DEMO_MODE §7.3 gains step 7, matching website's demo-wipe.yml.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

website/DEMO_MODE.md

@@ -403,6 +403,8 @@ current `rust-oxide`. **Done:** the org lead created the subuser and added the s
 6. Smoke check, after a bounded wait: `GET /api/v1/public/status` answers, and the Rust server
    reports a live connection. A rig that has not come back fails the run, and the site still
    serves with the server shown offline.
+7. `docker image prune -f`, whatever the run's outcome: a pull that found a newer image leaves the
+   old one untagged on the demo host every cycle. Dangling images only; nothing tagged or in use.
 
 ### 7.4 What a visitor sees