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/config.example.yaml

@@ -58,11 +58,6 @@ kubernetes:
 redis:
   url: redis://localhost:6379
 
-# Qdrant (for RAG vector search)
-qdrant:
-  url: http://localhost:6333
-  collection: gateway_memory
-
 # Iceberg (for durable storage via REST catalog)
 iceberg:
   catalog_uri: http://iceberg-catalog:8181