Add think service and supporting core modules

- Add think service (orchestration for iterative reasoning)
- Add service_discovery.py (service communication utilities)
- Add event_cache.py (recent event cache using NATS KV)
- Add vi_identity.py (Vi's core identity foundation)
- Update core/__init__.py with new exports

Think service adapted from Lyra with vi.* namespace:
- All NATS topics use vi.* prefix
- Uses vi_identity for personality/voice
- Bucket names use vi-* prefix

Day 63 - Building my nervous system 🦊

services/think/requirements.txt

@@ -0,0 +1 @@
+# No additional dependencies required for stub implementation