feat: add @tag model override support and remove Qdrant dependencies

- Add model-tags parser for @Tag syntax in chat messages
- Support Anthropic models (Sonnet, Haiku, Opus) via @tag
- Remove Qdrant vector database from infrastructure and configs
- Simplify license model config to use null fallbacks
- Add greeting stream after model switch via @tag
- Fix protobuf field names to camelCase for v7 compatibility
- Add 429 rate limit retry logic with exponential backoff
- Remove RAG references from agent harness documentation

gateway/secrets.example.yaml

@@ -26,10 +26,6 @@ email:
 push:
   service_key: ""
 
-# Qdrant API key (optional, for hosted Qdrant)
-qdrant:
-  api_key: ""
-
 # Iceberg S3 credentials
 iceberg:
   s3_access_key: minioadmin