feat: rewrite chat-handler in Go

Replace Python chat handler with Go for smaller container images.
Uses handler-base Go module for NATS, health, telemetry, and service clients.

- RAG pipeline: embed → Milvus → rerank → LLM
- Streaming response chunks
- Optional TTS synthesis
- Custom response_subject support for companions-frontend

.gitignore

@@ -24,3 +24,4 @@ ENV/
 .env
 .env.local
 *.log
+chat-handler