dsh-capability-facade: Providing a Narrow Tool Surface for DSH Plugin Authors

`dsh-capability-facade` is a reference plugin implementation for DeepSeek Harness (DSH), aimed at addressing the problem of overly broad plugin tool surfaces. It allows plugin authors to declare semantic capabilities and compose multiple underlying tools into a deterministic execution pipeline. A single model call can complete tasks that would otherwise require multiple round trips, reducing cost and improving determinism. The plugin requires underlying tools to be registered before the facade and does not hide already exposed tools. It is primarily used to stably combine multiple underlying tools into a high-level semantic operation.