#650 Internal Worker Bootstrap Hook

Auto-bootstrap on internal-worker startup. Flow: 1. Internal-worker starts 2. Check highway.platform_status table for 'singleton' row 3. If not exists or not initialized: a. Create minimal _platform tenant row (just for workflow execution) b. Submit _system.platform.bootstrap workflow to internal queue c. Internal-worker processes the workflow 4. Platform is now bootstrapped Modifications: - engine/services/activity_worker.py or new internal_worker.py - Add startup check before entering main loop - Idempotent: safe to run on multiple worker instances Parent: #639