dsh-package-manager: DSH Plugin Package Management Middleware

`dsh-package-manager` is the package management middleware for DSH. It implements declarative plugin installation and hot load/unload through the official `watchUserPatches` mechanism, without requiring a process restart. Its core features include single-pipeline hot updates, disable-on-toggle, version checks, unified CLI/Web/API interfaces, and pre-installation validation. It uses managed blocks and line-level diffs to manage dependencies and supports batch management of DSH plugins.