Agent Skills: cross-app-shared-auth

Blueprint for wiring many apps to one shared login plus a central accounts database, so a person has a single account across every app and access to each app accrues per app (opt-in, deny-by-default). Use this whenever onboarding a new app into a shared-auth family, setting up cross-subdomain SSO, building a central entitlement or "app access" database, adding per-app access gating, or when the user mentions one login for many products, shared accounts across apps, subdomain SSO, a central accounts service, app entitlements, or letting people sign up from any app OR a landing page and have it register centrally. Trigger even if the user does not say "shared auth" but describes a portfolio or family of apps that should share identity.