The /v1/responses endpoint used an in-memory OrderedDict that lost
all conversation state on gateway restart. Replace with SQLite-backed
storage at ~/.hermes/response_store.db.
- Responses and conversation name mappings survive restarts
- Same LRU eviction behavior (configurable max_size)
- WAL mode for concurrent read performance
- Falls back to in-memory SQLite if disk path unavailable
- Conversation name→response_id mapping moved into the store