dsh-token-vault: Local Credential Vault Plugin

dsh-token-vault is a secure credential management plugin for DeepSeek Harness (DSH). It addresses security risks of writing API tokens in plaintext or leaving them in history when calling APIs in DSH by storing tokens in the DSH credential vault, injecting them securely via environment variables, and preventing leakage. The plugin provides 7 tools that support creating, reading, updating, and deleting tokens, importing credentials for `gh`/`npm`, and securely running commands. Its security design spans three layers: storage (atomic writes), usage (no echo), and control (preventing agents from printing credentials). Users can install it via a command and manage credentials in DSH settings, making it suitable for securely invoking external tools such as `gh` and `npm` within the DSH environment.