dsh-tesseract-ocr: Connecting text models to local image OCR

`dsh-tesseract-ocr` is a DeepSeek Harness (DSH) plugin designed to address the limitation where text models cannot process image inputs. By invoking the local Tesseract OCR engine to recognize text in attached images, the plugin sends the recognized plain text to the model API. By default, it retains the original image bytes locally without uploading them to the model provider, thereby avoiding changes to the model's input modality or the transmission of sensitive data. Core features include support for text models to process image attachments, local OCR recognition, sending only text by default, an optional visual model passthrough mode (`passthrough: true`), and a fail-closed security mechanism that rejects image attachments when the plugin is not loaded. It is suitable for scenarios where DSH text models are used and require image-text Q&A.