docs(adr): propose ADR-0105 — group tenancy posture and first-class org scope (#3499)

Three tenancy postures (single | group | isolated), an accessible_org_ids
union wall preserving ADR-0095 W1/W2, P0 findings F1 (authored-RLS
stripping in collectRLSPolicies) and F2 (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.

Includes an empty changeset (docs-only, releases nothing).

Author: os-zhuang

.changeset/adr-0105-group-tenancy-posture.md

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+Docs-only: propose ADR-0105 (group tenancy posture — organization scope as a
+first-class authorization dimension). Releases nothing.