Add ATR as a portable rule-pack scanner backend alongside Cisco AI Defense

The registry already scans MCP servers at registration with Cisco AI Defense. Offering a second, MIT-licensed, self-hostable rule source would let operators who can't use a commercial backend still get coverage.

ATR (Agent Threat Rules) is exactly that — an open detection-rule standard. I can PR an ATR scanner backend behind the same registration-scan interface, as a complement to the Cisco one. Want it?