dsh-client-masquerade: Adding Claude Code / Codex Client Masquerade to DSH's llm-pi-ai Routing

`dsh-client-masquerade` is a DeepSeek Harness (DSH) plugin designed to address client identity recognition and request-body fingerprint verification issues when connecting through `llm-pi-ai` to an intermediate gateway. This plugin writes preset request headers such as `user-agent` into the provider configuration, ensuring they are sent as-is by the adapter without being stripped or overwritten. For scenarios requiring stricter verification, it can also inject Claude Code fingerprints (such as `metadata.user_id`) into the Anthropic protocol request body to pass the gateway's identity authentication. The plugin supports one-click application, clearing, or switching between Claude Code, Codex, and custom masquerade modes, and provides a settings page and model tools.