dsh-fabric: Refactoring DSH runtime extensions into a set of plugin primitives

dsh-fabric is an extension plugin for DeepSeek Harness (DSH) designed to address the issue of scattered runtime extension points. Through a declarative DSL, a unified `fabric` Service, and two model tools, the plugin converges and unifies the management of over a dozen types of extension seams, such as tools, skills, commands, prompt segments, and event listeners. Core functions include providing Service APIs like `register`, `list`, and `graph` for plugin code to register and manage extensions; offering the `fabric_extend` tool to support the dynamic registration of pure declarative extensions (e.g., `promptVariable`) in conversations; and providing the `fabric_inspect` tool and a read-only Web route `/fabric`.