dsh-freeroute: Free Model Aggregation and Failover Plugin for DSH

The `dsh-freeroute` plugin in DSH (DeepSeek Harness) addresses the pain points developers face when invoking paid models or managing multiple free model sources through a plug-in architecture. It registers the freeroute model provider and supports automatic failover, multi-key rotation, local OpenAI-compatible endpoints, and expired-key handling. It simplifies configuration, provides disaster-recovery capabilities, and enables users to conveniently aggregate legitimate free model tiers for testing and use.