dsh-api-client: An Agent-Native HTTP Client for DeepSeek Harness

`dsh-api-client` is a native plugin for DeepSeek Harness, designed to address the lack of a unified audit trail and context sharing when debugging HTTP requests. It natively integrates Human UI—a Postman-grade debugging experience—with Agent Tools and supports importing Postman Collections. Core features include request editing, response viewing, and environment variable management; exposing `api_client_*` tools for Agent invocation; built-in SSRF protection and sensitive data masking; and policy configuration directly in the DSH interface. The plugin enables developers to share the same execution engine during debugging and allows Agents to invoke HTTP tools directly, ensuring end-to-end security from debugging to execution.