docs: add ATR (Agent Threat Rules) to the list of tools supporting Sigma

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Summary of the Pull Request

Adds ATR (Agent Threat Rules) to the list of projects and products that use or integrate Sigma rules in the README.

ATR is an MIT-licensed open detection rule format for AI agent security threats. Authoring shape is similar to Sigma but targeted at LLM input/output, MCP tool calls, and agent context windows. The reference CLI exports ATR rules to Sigma format via atr convert sigma, so any Sigma-consuming pipeline can ingest ATR-derived rules directly.

Repository: https://github.com/Agent-Threat-Rule/agent-threat-rules
License: MIT
DOI: 10.5281/zenodo.19178002

Single-line addition to keep the change minimal and easy to review. Happy to adjust placement, wording, or formatting to match maintainer preference.

Changelog

docs: add ATR (Agent Threat Rules) to the list of tools supporting Sigma

Example Log Event

Fixed Issues

SigmaHQ Rule Creation Conventions

• If your PR adds new rules, please consider following and applying these conventions