dsh-wsl-secret: DeepSeek Harness WSL Plugin

`dsh-wsl-secret` is a plugin for the DeepSeek Harness (DSH) environment that securely manages application-layer API keys. It supports reading keys in `pass` and `age` formats and provides features such as listing, inspecting, decrypting, and loading keys into environment variables. Before use, the `pass` and `age` tools must be installed in the WSL/Linux environment, and the `allowPrefixes` whitelist must be configured in the DSH profile to activate it. By default, the plugin hides key contents. It is recommended to use the `secret_to_env` command to inject keys into environment variables for in-process access, avoiding direct plaintext exposure.