feat: add ATR agent threat scanning MCP server

[eeee2345]

Summary

Add an ATR (Agent Threat Rules) MCP server for regex-based AI agent threat detection. Unlike other servers in this hub that wrap external binaries, this server is pure Python with zero external tool dependencies.

Tools

Tool	Description
atr_scan_text	Scan arbitrary text against 20 ATR rules. Supports context-aware filtering (skill mode excludes high-FP rules).
atr_scan_mcp_config	Scan a full MCP config JSON — extracts each server's command/args and scans for threats. Env var values are redacted to prevent credential-exposure false positives.
atr_list_rules	List loaded rules with optional category filter.
atr_rule_info	Get details for a specific rule by ID.

Threat categories covered (20 rules)

Prompt injection (3), jailbreak (2), system prompt override/extraction (2), tool poisoning (2), data exfiltration (2), reverse shell (1), credential exposure (1), privilege escalation (1), cross-agent manipulation (1), hidden instructions (1), social engineering (1), path traversal (1), code injection (1), encoded payload (1).

This is a curated subset of 20 high-precision rules from the ATR standard (108 rules total, MIT licensed).

Files

File	Description
agent-security/atr-mcp/server.py	MCP server implementation (4 tools)
agent-security/atr-mcp/rules.json	20 bundled ATR rules
agent-security/atr-mcp/Dockerfile	Alpine-based, non-root, following project template
agent-security/atr-mcp/README.md	Usage docs
agent-security/atr-mcp/requirements.txt	mcp, pydantic (2 deps)
docker-compose.yml	Added atr-mcp service
tests/test_mcp_servers.py	Added test registration
examples/claude-desktop-config.json	Added config example

Key design choices

• Context-aware scanning: context="skill" excludes 4 high-FP rules for SKILL.md content
• Deduplication: One finding per rule per scan (no duplicates)
• Env var redaction: MCP config scanner redacts env values before scanning
• Pure Python: No external binary, read_only: true Docker filesystem, 256M memory limit

Test plan

• Server registers 4 tools via @app.list_tools()
• Added to test_mcp_servers.py parametrized tests
• Dockerfile builds successfully
• CI validation