dsh-plugin-mall: Add a GitHub Plugin Marketplace to dsh

dsh-plugin-mall is a community-maintained dsh plugin marketplace tool designed to streamline the process of discovering, validating, installing, and updating dsh plugins from GitHub. By searching for repositories tagged with `topic:dsh-plugin`, it automatically fetches and validates the manifest in `package.json`, thereby eliminating the tedious steps of manual searching and compatibility checks. The tool provides a plugin marketplace entry in the settings page of the dsh web UI and includes five general-purpose agent tools. Its core security mechanisms include static compatibility badge scanning during browsing, isolated preflight checks before installation (which disable scripts, install to a temporary directory, and compare live profiles to block hard conflicts), and anti-squash...