dsh-allrouter: Unified Integration of AllRouter's Model Providers

The `dsh-allrouter` plugin achieves one-click switching across multiple models by registering AllRouter as a unified provider for DeepSeek Harness (DSH). By maintaining only one API key, it enables the use of multiple models such as Kimi K3, Claude, and GPT in DSH, with real token-based billing. After installation, configure the key via an environment variable and switch directly in DSH; no local tool code is required. It offers security features including zero-permission access, direct pricing, and no data storage, making it especially suitable for agent development scenarios that require frequent model switching.