Add model card

This PR adds a model card for the ShadowMem judge model, linking to the paper [Safeguarding LLM Agents against Long-Horizon Threats via Shadow Memory](https://huggingface.co/papers/2605.03228) and the [GitHub repository](https://github.com/ZJUWYH/ShadowMem). It also sets the `library_name` (transformers) and `pipeline_tag` (text-classification) metadata to enable easier discovery and usage.

README.md

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+---
+library_name: transformers
+pipeline_tag: text-classification
+---
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+This repository contains the judge model described in [Safeguarding LLM Agents against Long-Horizon Threats via Shadow Memory](https://huggingface.co/papers/2605.03228).
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+ShadowMem is a defensive framework that maintains a dedicated, safety-focused agentic memory — inspired by the "shadow stack" abstraction in systems security — to distill and retain safety-critical context across an agent's full execution trajectory, and uses this shadow memory to proactively assess the risk of pending actions prior to their execution. This model is a fine-tuned Qwen3-based language model used as the safety judge that produces those risk assessments.
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+Code: https://github.com/ZJUWYH/ShadowMem