Add mpp.directory skill

[sam00101011]

Summary

• add a new mpp-directory skill for registry discovery, routing, and signals
• document the core public API routes and agent-facing surfaces
• list the skill in the Bankr skills index

Validation

• curl -fsSL https://mpp.directory/v1/public/endpoints?limit=2
• curl -fsSL https://mpp.directory/v1/signals
• git diff --check