feat(hub): add global nestable member group hierarchy

Global, unlimited-depth member groups managed by the platform super
admin (requirement 3.1-3.3). Stores membership + nesting only, never
permission data; exposes user -> ancestor-closed group set.

- MemberGroup: soft delete via archivedAt; parentId FK RESTRICT.
  Deleting a group cascade-soft-deletes its whole subtree as an
  application operation, not a DB cascade.
- MemberGroupMembership: user<->group many-to-many, revokedAt soft
  delete, user/group indexed for resolution hot path.
- MemberGroupClosure: transitive closure (depth-0 self rows) for
  one-join ancestor/descendant resolution; maintained on
  create/reparent with a cycle guard.

Permission side (GROUP principal, FOLDER resource, grant inheritance)
is deferred. This principal is deliberately not org-scoped and will
need ADR-0028 to supersede the ADR-0020 cross-org invariant before the
GROUP principal ships.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hub/prisma/migrations/20260723120000_member_group_hierarchy/migration.sql

@@ -0,0 +1,70 @@
+-- Global, unlimited-depth member group hierarchy (requirement 3.1-3.3).
+-- Managed only by the platform super administrator; deliberately NOT
+-- org-scoped. Stores membership + nesting only, never permission data.
+-- Deletion is soft (archivedAt / revokedAt markers); a group delete
+-- cascade-soft-deletes its whole subtree as an application operation.
+-- The permission side (GROUP principal, FOLDER resource, grant inheritance)
+-- is intentionally deferred to a later migration.
+
+-- CreateTable
+CREATE TABLE "MemberGroup" (
+    "id" TEXT NOT NULL,
+    "parentId" TEXT,
+    "name" TEXT NOT NULL,
+    "description" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+    "archivedAt" TIMESTAMP(3),
+
+    CONSTRAINT "MemberGroup_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "MemberGroupMembership" (
+    "id" TEXT NOT NULL,
+    "groupId" TEXT NOT NULL,
+    "userId" TEXT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "revokedAt" TIMESTAMP(3),
+
+    CONSTRAINT "MemberGroupMembership_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "MemberGroupClosure" (
+    "ancestorId" TEXT NOT NULL,
+    "descendantId" TEXT NOT NULL,
+    "depth" INTEGER NOT NULL,
+
+    CONSTRAINT "MemberGroupClosure_pkey" PRIMARY KEY ("ancestorId", "descendantId")
+);
+
+-- CreateIndex
+CREATE INDEX "MemberGroup_parentId_archivedAt_idx" ON "MemberGroup"("parentId", "archivedAt");
+
+-- CreateIndex
+CREATE INDEX "MemberGroupMembership_userId_revokedAt_idx" ON "MemberGroupMembership"("userId", "revokedAt");
+
+-- CreateIndex
+CREATE INDEX "MemberGroupMembership_groupId_revokedAt_idx" ON "MemberGroupMembership"("groupId", "revokedAt");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "MemberGroupMembership_groupId_userId_revokedAt_key" ON "MemberGroupMembership"("groupId", "userId", "revokedAt");
+
+-- CreateIndex
+CREATE INDEX "MemberGroupClosure_descendantId_idx" ON "MemberGroupClosure"("descendantId");
+
+-- AddForeignKey
+ALTER TABLE "MemberGroup" ADD CONSTRAINT "MemberGroup_parentId_fkey" FOREIGN KEY ("parentId") REFERENCES "MemberGroup"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "MemberGroupMembership" ADD CONSTRAINT "MemberGroupMembership_groupId_fkey" FOREIGN KEY ("groupId") REFERENCES "MemberGroup"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "MemberGroupMembership" ADD CONSTRAINT "MemberGroupMembership_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "MemberGroupClosure" ADD CONSTRAINT "MemberGroupClosure_ancestorId_fkey" FOREIGN KEY ("ancestorId") REFERENCES "MemberGroup"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "MemberGroupClosure" ADD CONSTRAINT "MemberGroupClosure_descendantId_fkey" FOREIGN KEY ("descendantId") REFERENCES "MemberGroup"("id") ON DELETE CASCADE ON UPDATE CASCADE;

hub/prisma/schema.prisma

@@ -193,6 +193,7 @@ model User {
   heldLocks                    ProjectAgentLock[]                   @relation("lockHolder")
   feishuBindings               ProjectGroupBinding[]                @relation("bindingCreator")
   teamMemberships              TeamMembership[]
+  memberGroupMemberships       MemberGroupMembership[]
   externalPrincipalMemberships ExternalPrincipalMembership[]
   permissionGrants             PermissionGrant[]                    @relation("grantCreator")
   roleTriggerGrants            RoleTriggerGrant[]                   @relation("roleGrantCreator")
@@ -393,6 +394,67 @@ model TeamExternalBinding {
   @@index([teamId, revokedAt])
 }
 
+// --- Member groups (global, nestable authorization principal) ------------
+
+/// Global, unlimited-depth member group. Managed only by the platform super administrator (ADR-0023);
+/// Deletion is soft: `archivedAt` is a marker.
+/// Deleting a group cascade-soft-deletes its whole subtree — an application
+/// operation (walk the subtree, stamp archivedAt), not a DB constraint.
+model MemberGroup {
+  id          String    @id @default(cuid())
+  parentId    String?
+  name        String
+  description String?
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+  archivedAt  DateTime?
+
+  parent       MemberGroup?            @relation("groupTree", fields: [parentId], references: [id], onDelete: Restrict)
+  children     MemberGroup[]           @relation("groupTree")
+  memberships  MemberGroupMembership[]
+  asAncestor   MemberGroupClosure[]    @relation("ancestor")
+  asDescendant MemberGroupClosure[]    @relation("descendant")
+
+  @@index([parentId, archivedAt])
+}
+
+/// User↔group many-to-many; a user may belong to multiple groups. Soft delete
+/// via `revokedAt` (same pattern as TeamMembership) allows re-adding a removed
+/// member. `userId, revokedAt` index is the resolution hot path: fetch a user's direct groups.
+model MemberGroupMembership {
+  id        String    @id @default(cuid())
+  groupId   String
+  userId    String
+  createdAt DateTime  @default(now())
+  revokedAt DateTime?
+
+  group MemberGroup @relation(fields: [groupId], references: [id], onDelete: Cascade)
+  user  User        @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([groupId, userId, revokedAt])
+  @@index([userId, revokedAt])
+  @@index([groupId, revokedAt])
+}
+
+/// Transitive closure of the MemberGroup tree. Every group has a depth=0
+/// self row. Turns ancestor/descendant resolution into one indexed join
+/// instead of a recursive CTE; maintained only on create / reparent (rare
+/// super-admin ops, so the write cost is amortized against hot reads). On soft
+/// delete the closure rows are retained; resolution filters by
+/// MemberGroup.archivedAt. Reparent must reject a new parent inside the moved
+/// subtree (cycle guard).
+model MemberGroupClosure {
+  ancestorId   String
+  descendantId String
+  depth        Int
+
+  ancestor   MemberGroup @relation("ancestor", fields: [ancestorId], references: [id], onDelete: Cascade)
+  descendant MemberGroup @relation("descendant", fields: [descendantId], references: [id], onDelete: Cascade)
+
+  @@id([ancestorId, descendantId])
+  @@index([descendantId])
+}
+
 /// Locally synchronized Feishu external principal membership.
 model ExternalDirectoryConnection {
   id               String    @id @default(cuid())