Add Nunchi relay binding and connect skill

[sdb001]

Summary

• Adds a dependency-free deploy/nunchi-relay service for pairing external agents with Nunchi account sessions.
• Adds OpenClaw Railway binding support with heartbeat metadata, bind-session polling, auth-code confirmation, and template-version defaults.
• Adds the nunchi-connect skill prompt that users can give to existing agents so they can bind back to Agent Studio.
• Documents the new skill in the README.

Verification

• Relay smoke checks passed locally against /health, POST /agents/bind, GET /agents/bind/:sessionId, and POST /agents/bind/:sessionId/confirm.
• python3 -m cli.main skills list --json was attempted but blocked by missing local dependency typer.