dsh-plugin-graph: DSH Plugin Dependency Graph and Conflict Detection Engine

`dsh-plugin-graph` is an ecosystem-level infrastructure plugin in the DSH ecosystem, designed to address challenges in plugin dependency management and conflict troubleshooting. Through static source code analysis, it provides four conflict detection capabilities (tools, services, pipelines, and event listeners) and dependency graph construction features (including cyclic dependency and missing dependency detection), while also supporting version compatibility verification and installation preflight checks. It supports CLI and CI integration, runs safely and efficiently as a pure function, and is suitable for use during the plugin development and maintenance phases, helping developers avoid potential risks.