Phase 2 of Core DI migration:
- Add *core.Core field + SetCore() to PrepSubsystem and monitor.Subsystem
- Register agentic/monitor/brain as Core services with lifecycle hooks
- Mark SetCompletionNotifier and SetNotifier as deprecated (removed in Phase 3)
- Fix monitor test to match actual event names
- initServices() now wires Core refs before legacy callbacks
Co-Authored-By: Virgil <virgil@lethean.io>
