feat(config): add runtime_fallback and fallback_models schema

Add configuration schemas for runtime model fallback feature:
- RuntimeFallbackConfigSchema with enabled, retry_on_errors,
  max_fallback_attempts, cooldown_seconds, notify_on_fallback
- FallbackModelsSchema for init-time fallback model selection
- Add fallback_models to AgentOverrideConfigSchema and CategoryConfigSchema
- Export types and schemas from config/index.ts

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/chat-message.ts

@@ -54,6 +54,7 @@ export function createChatMessageHandler(args: {
     }
 
     await hooks.stopContinuationGuard?.["chat.message"]?.(input)
+    await hooks.runtimeFallback?.["chat.message"]?.(input, output)
     await hooks.keywordDetector?.["chat.message"]?.(input, output)
     await hooks.claudeCodeHooks?.["chat.message"]?.(input, output)
     await hooks.autoSlashCommand?.["chat.message"]?.(input, output)