feat: add NATS service discovery tools #58

Summary

Add library and command-line helpers that query the NATS Service API for service information, statistics, and endpoint subject maps.

Acceptance criteria

• Discover service INFO, PING, and STATS responses through configurable NATS Service API subjects.
• Provide command-line tools for service information, service lists, statistics, and endpoint subject maps.
• Keep existing Leopard behavior unchanged and cover the new workflows with integration tests.