docs(adr): propose ADR-0105 — group tenancy posture and first-class org scope

Synthesizes the group/multi-plant architecture review: three tenancy
postures (single | group | isolated), an accessible_org_ids union wall
preserving ADR-0095 W1/W2, P0 fixes for authored-RLS stripping
(collectRLSPolicies substring match) and uncontained wildcard VAMA in
wall-less deployments, the cut-line doctrine for organizations vs
business-unit trees, managed-table extension fields via the ADR-0092
whitelist, and a BU-subtree-to-organization promotion path along the
single/group/isolated spectrum.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CbjR5BaHy9tGwUR8DoJA36

Author: claude