dsh-git-credentials: Keep Git platform API token values out of model context

`dsh-git-credentials` is a DeepSeek Harness plugin designed to address the security challenge of preventing tokens from entering the model context when calling APIs of Git platforms like GitLab and GitHub within DSH agents. The plugin decouples credential management from model logic, allowing the model to process only business parameters, while token values are decrypted by the plugin from local AES-256-GCM encrypted storage and directly used in the outbound Authorization headers. Its core features include limiting tool invocation scope by Provider (e.g., `gitlab_*`, `github_*`), and providing a Web settings panel that supports CRUD operations for sites and hot token rotation, with changes taking effect immediately without requiring a restart.