chore: drop legacy PlatformRoleAssignment model

ADR-0023 / Spec.System.PlatformAdministration pins the platform
administrator as a separate identity/session/audit control plane,
intentionally not modeled in alpha (ADR-0025). The legacy
PlatformRoleAssignment / PlatformRole{ADMIN,TEACHER} table had no runtime
reader (no guard, route, or service queried it for an authorization
decision) and ADR-0023 requires it to be replaced before the platform
panel ships.

Drop the model, the PlatformRole enum, the User.platformRoles relation,
and the migration. Stop seeding platformRoles in externalSync principal
ingestion and the integration test helper. Update the doc comments on
OrganizationMembership and PermissionRole to point at the platform-admin
control plane instead of the dropped model.

The 20260709180000_organization_tenant_root backfill only referenced
PlatformRoleAssignment in a one-time INSERT...SELECT; no persistent
object references it, so dropping the table is safe after that migration.

hub/prisma/schema.prisma

@@ -58,8 +58,9 @@ enum OrganizationStatus {
   ARCHIVED
 }
 
-/// Org-scoped platform role. Distinct from project PermissionRole and from
-/// global PlatformRoleAssignment, which is reserved for SaaS/platform control.
+/// Org-scoped membership role. Distinct from project PermissionRole and from
+/// the platform administrator surface (ADR-0023 / Spec.System.PlatformAdministration),
+/// which is a separate control plane not modeled in alpha (ADR-0025).
 model OrganizationMembership {
   id             String                 @id @default(cuid())
   organizationId String
@@ -170,7 +171,6 @@ model User {
   createdAt    DateTime @default(now())
   updatedAt    DateTime @updatedAt
 
-  platformRoles                PlatformRoleAssignment[]
   organizationMemberships      OrganizationMembership[]
   createdProjects              Project[]                            @relation("projectCreator")
   requestedRuns                AgentRun[]                           @relation("runRequester")
@@ -311,26 +311,6 @@ model ProviderCredentialVersion {
   @@index([createdByUserId])
 }
 
-/// Platform-level role (admin/teacher). Distinct from ADR-0004 PermissionRole.
-/// `admin` is the only override path for force-release (spec RequiresAdmin).
-model PlatformRoleAssignment {
-  id        String       @id @default(cuid())
-  userId    String
-  role      PlatformRole
-  createdAt DateTime     @default(now())
-  revokedAt DateTime?
-
-  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
-
-  @@index([userId, revokedAt])
-  @@index([role, revokedAt])
-}
-
-enum PlatformRole {
-  ADMIN
-  TEACHER
-}
-
 /// ADR-0019: typed principals for permission grants and actor resolution.
 /// USER and Feishu external principals use connection-scoped opaque ids, never
 /// raw provider-local ids; TEAM uses Hub Team.id.
@@ -647,8 +627,9 @@ model ProjectAgentLock {
 // --- Permission grants & settings (ADR-0004) ----------------------------
 
 /// ADR-0004 PermissionRole: read ⊂ edit ⊂ manage (capability lattice).
-/// Distinct from PlatformRole. Force-release is admin-only, outside this
-/// lattice (spec RequiresAdmin).
+/// Force-release is platform-admin-only, outside this lattice (spec
+/// RequiresAdmin); platform admin is a separate control plane (ADR-0023),
+/// not modeled in alpha (ADR-0025).
 enum PermissionRole {
   READ
   EDIT