dsh-plugin-dedupe: Add a plugin deduplication check for DSH profiles

## Introduction When using DSH, plugins are generally installed in the profile directory and managed uniformly by the profile's `package.json`. When installed in large numbers, a problem arises: the same plugin may be declared twice—for example, once from `github:` and once from a local `file:` source. Even with the same package name but different origins, multiple copies of the same-named package will coexist in `node_modules`. This can waste disk space or, more critically, cause the runtime to load the wrong version, which is time-consuming to troubleshoot. `dsh-plugin-dedupe` solves this problem by scanning the profile before installation and blocking the installation process if duplicate declarations are detected. The following sections introduce its features, installation, and usage. ## What is dsh-plugin