dsh-credential-handoff: Enable agents to request credentials without embedding keys into the model context.

`dsh-credential-handoff` is an MIT-licensed plugin designed for DSH plugin-based scenarios to address privacy risks where API keys may enter session logs or model contexts when an agent requests them. It registers a `request_credential` tool to collect credentials via a browser dialog and stores them using the DSH credential service in a write-only manner. The tool call returns only fixed status metadata (such as credential references and configuration status), ensuring that key values are not returned to the model context. The plugin supports HTTPS submissions and plaintext HTTP over local loopback, while preventing non-local plaintext transmission. Each session allows only one pending request, and the dialog expires after ten minutes. Credential references follow shell-style naming rules.