dsh-file-resource: Local file input plugin for DeepSeek Harness Web

In DeepSeek Harness Web, while image attachments have a native workflow, there is no ready-made entry point for documents. To allow the model to view a PDF report or an XLSX spreadsheet, the common practice is to copy the text and paste it into the input box. However, long documents are easily truncated, and table structures and formatting are often lost. Building a custom upload and parsing pipeline also involves handling complexities such as storage, format support, and context usage. DSH's philosophy is "everything is a plugin," so file input can also be delegated to a plugin. The `dsh-file-resource` introduced below accomplishes exactly this: it adds an `attach` entry at the top of the existing `+` menu in the input box. Documents are parsed locally into session-isolated private resources, which the model reads on demand via a restricted read tool.