dsh-openapi: Adding OpenAPI Discovery and Secure Invocation Tools to DeepSeek Harness

dsh-openapi is a plugin for DeepSeek Harness, designed to address issues such as inconsistent parameters, uncontrolled credentials, and excessive scope when models directly call APIs. This plugin indexes OpenAPI 3.x documents and provides three core tools: `openapi_list` (discovery and search operations), `openapi_describe` (viewing details), and `openapi_call` (restricted invocation). Its core features include strict security boundaries: by default, only GET/HEAD requests are allowed, access to local and private network IPs is blocked, response body sizes are limited, and sensitive response headers are shielded, while redirect checks are supported. Credential management is implemented through environment variable mapping, ensuring that keys are not directly exposed in configurations or outputs. Installation requires the use of the `dsh plugin` command.