dsh-tavily: Tavily Web Search Plugin for DeepSeek Harness

dsh-tavily is a Tavily web search plugin for DeepSeek Harness (DSH), designed to address the high cost of switching search sources. This plugin enables seamless switching between the official DeepSeek and Tavily through a settings card toggle, eliminating the need for repeated uninstallation and reinstallation. Core features include: support for a keyless mode without a Key and an account profile with a Key (Bearer authentication); a connectivity test function that can genuinely invoke Tavily to verify status; intelligent handling of timeouts and invalid results; secure storage of credentials (Key/toggle) in `credentials`, without writing to settings files; and the use of an independent namespace to avoid overriding official settings. Installation first requires adding the plugin via the `dsh plugin` command, then enabling the toggle in settings and