Add RouterBase model gateway skill

[zenlee123]

Summary

• Add a RouterBase model gateway skill for OpenAI-compatible routing, fallback planning, and API key handling
• Register it in the full marketplace and tooling marketplace
• Add README listing, usage examples, and direct download link

Notes

• External contribution from zenlee123 for RouterBase-related workflows
• Uses only placeholder environment variables; no real API keys or private data
• Includes a natural source link to routerbase in the skill and README

Validation

• jq empty .claude-plugin/marketplace.json .claude-plugin/marketplace-tooling.json
• git diff --check
• bash scripts/build-skills.sh routerbase-model-gateway
• targeted secret scan for API keys/tokens