feat(phase-3): workstream 1 — Multi-Tenancy

Introduces full multi-tenant organization model to AgentIdP:

Schema:
- 6 migrations: organizations + organization_members tables; organization_id FK
  added to agents, credentials, audit_logs; PostgreSQL RLS policies on all three
  tables; system org seed + backfill

API:
- 6 new /api/v1/organizations endpoints (CRUD + members) gated by admin:orgs scope
- OPA scopes.json updated with 6 new org endpoint → admin:orgs mappings

Implementation:
- OrgRepository, OrgService, OrgController, createOrgsRouter
- OrgContextMiddleware: sets app.organization_id session variable so RLS enforces
  per-request org isolation at the database layer
- JWT payload extended with organization_id claim; auth.ts backfills org_system
  for backward-compatible tokens
- New error classes: OrgNotFoundError, OrgHasActiveAgentsError, AlreadyMemberError

Tests: 373 passing, 80.64% branch coverage, zero any types

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/services/OAuth2Service.ts

@@ -185,6 +185,7 @@ export class OAuth2Service {
       client_id: clientId,
       scope,
       jti,
+      organization_id: agent.organizationId ?? 'org_system',
     };
 
     const accessToken = signToken(payload, this.privateKey);