dsh-tool-schema: DSH JSON Schema Validation Tool Plugin

`dsh-tool-schema` is a DSH local JSON Schema validation plugin released under the MIT License, designed to address data validation, error localization, and Schema explanation issues within complex nested structures via a single tool call. The plugin provides four core actions—`validate`, `paths`, `explain`, and `normalize`—used for comprehensive validation, rapid failure path localization, static constraint explanation, and safe default value application prior to validation. Emphasizing security and stability, the plugin strictly adheres to zero network access and zero dynamic code execution, operating within fixed resource limits such as data size (256 KiB), nesting depth (64 layers), and node count to prevent performance abuse. Its validation kernel adopts a pure function style and supports local `$ref`.