#21 Description via flags/env (header + og)

Context: Owner review round — header shows no description; must come from config (flags/env only, no config file per doctrine). Tasks:

1. --description S flag + HU13_DESCRIPTION env (flag wins, then env, then fallback: first line of wiki README? NO — fallback: empty, header hides description line). Thread through build into header + og:description + api (add description field to a new api/meta.json? or index only — decide: header + og + feed subtitle; keep api unchanged to avoid breaking agents).
2. Entrypoint: pass HU13_DESCRIPTION through. Dockerfile ENV default empty.
3. Demo: description text lives in DEPLOY (env on the live container), not the repo. Supervisor sets -e HU13_DESCRIPTION="..." on hu13-live.
4. Tests: description shown escaped (hostile <script> in description renders as text), absent description hides the line (no empty <p>), og:description matches. Success rubric:

• Flag/env/fallback chain tested; escaping verified; entrypoint passes env. Swarm: Owlette. Branch m6/description. Small, parallelizable.