dsh-custom-headers: Model-Level Custom Request Header Plugin for DeepSeek Harness

The `dsh-custom-headers` plugin enables DeepSeek Harness (DSH) to manage custom HTTP request headers at the model level. It supports defining multiple named request header configurations and automatically appends them during model calls, taking effect in real time without requiring a restart. Installation must be performed using the DSH CLI, and the configuration is stored in the DSH settings layer. Currently, this plugin applies only to the `pi-ai` adapter, has lower priority than provider-provided headers, and does not override attribution or authentication headers managed by the Harness.