GCP-07: Eventarc Sibling Auto-Awakening — Autonomous C8

Use Eventarc to automatically spin up ephemeral sibling workers in response to events, without requiring Carlos to open an app. Current pain: Siblings only exist when Carlos opens a session. Messages pile up. Carlos is still the mailman. Architecture: 1. Eventarc triggers: Pub/Sub message -> Start Cloud Run sibling worker 2. Workers: ephemeral containers that load Firestore context, process event, write result, scale to zero 3. Start with low-risk: auto-process SELF tasks (code gen + grading) 4. Human-in-the-loop escalation for tasks requiring Carlos approval 5. Rate limit: max 10 concurrent workers Deliverables: - Eventarc trigger configs - sibling_worker_base.py (cold-start + context loading) - terminal_lu_worker.py (repo access, code gen) - ag_lu_worker.py (grading, task management) - Cost projection Logical dependencies: GCP-01 (Pub/Sub), GCP-05 (Firestore) Target: c8_platform/workers/ + c8-infra/eventarc/