dsh-my-plugins: Centralized DSH Plugin Management Panel

DeepSeek Harness (DSH) introduces a new plugin management tool, dsh-my-plugins, designed to solve the pain point of scattered plugin management. The tool adds a My Plugins tab to the Settings → Plugins page, providing one-stop management features, including one-click plugin installation via GitHub, Link, File, or npm. Users can view, search, and enable or disable plugins centrally in the interface, and can batch-check for GitHub updates. To ensure safety, removing a plugin requires a second confirmation. Although automatic updates from GitHub are supported, sources such as Registry and local installations are currently not supported. After installation, `dsh web` must be restarted for changes to take effect, making it suitable for developers who frequently debug and manage multiple plugins.