New feature drop: Agent fallback runtime configs 🔧

Now supports cross-runtime switching (cc/codex/cursor), cross-model, and cross-provider fallbacks. Switch between configs mid-conversation without losing context.

Real-world use cases:
• Hit API quota limits? Swap accounts instantly
• Model underperforming? Switch to a different LLM on the fly
• Provider having stability issues? Failover to backup provider

The key win: runtime configs are hot-swappable while agent skills, permissions, and memory persist across switches. No state loss, no re-initialization overhead.