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/.env.example

@@ -38,10 +38,6 @@ SANDBOX_STORAGE_CLASS=standard
 # Redis (for hot storage and session management)
 REDIS_URL=redis://localhost:6379
 
-# Qdrant (for RAG vector search)
-QDRANT_URL=http://localhost:6333
-QDRANT_API_KEY=  # optional, leave empty for local dev
-
 # Iceberg (for durable storage via REST catalog)
 ICEBERG_CATALOG_URI=http://iceberg-catalog:8181
 ICEBERG_NAMESPACE=gateway