Document the self-documenting Foundry pattern

[jmchilton]

What changed

• expand the existing Design Records page into the single home for the self-documenting Foundry pattern
• add a responsive content/meta/ diagram, the typed directory contract, honest-gap guidance, and a concise adoption path
• document the existing review-design-docs plugin skill and its change-routing behavior
• update the Pattern landing page and related guidance to point to the consolidated authority
• remove stale deterministic/model-boundary language so casting remains deterministic

Why

The proposed self-documentation overview and the existing Design Records reference were explaining the same pattern. Keeping both would create competing authorities and violate the ownership discipline the pages advocate. This folds the useful new material into the established content/pattern/design-records.md path that the review skill already treats as canonical.

Impact

Readers get one page that explains both why a Foundry documents itself and how each record is owned and maintained. Existing links and the review skill keep their canonical path.

Checks

• npm test — 22 tests passed
• npm run typecheck — no errors; one pre-existing unused-variable hint
• npm run build — 28 pages built, 21 wiki-link targets checked
• git diff --check