dsh-file-viewer: Read-only file preview in DeepSeek Harness

`dsh-file-viewer` is a read-only file preview plugin for DeepSeek Harness, maintained by `liguobao` (MIT License, v0.3.1). It allows direct previewing of files such as images, PDFs, CSVs, Markdown, JSON, YAML, source code, and large logs directly within the web page, without relying on external applications. Strictly adhering to the "Preview ≠ Execute" principle, it prohibits the execution of scripts, macros, or commands to ensure security. The plugin supports a tiered loading strategy for large files (<5MB read entirely, 5-50MB chunked, >50MB load header only) and provides the `fileViewer.openFile` API, supporting opening local paths, `artifact://` locators, specifying line numbers, and forcing a specific renderer.