Get the authenticated user's effective org-context envelope

Returns the active org context, the full membership list, and any user preferences that affect the dashboard. Per ADR-044 Piece 1 / migration 164. The dashboard's authoritative endpoint for "who am I, which org am I in, what are my memberships." active falls back to the most-recently-accepted multi-user org, then the personal org; null only if the user has zero memberships.