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/event.ts

@@ -42,6 +42,7 @@ export function createEventHandler(args: {
     await Promise.resolve(hooks.rulesInjector?.event?.(input))
     await Promise.resolve(hooks.thinkMode?.event?.(input))
     await Promise.resolve(hooks.anthropicContextWindowLimitRecovery?.event?.(input))
+    await Promise.resolve(hooks.runtimeFallback?.event?.(input))
     await Promise.resolve(hooks.agentUsageReminder?.event?.(input))
     await Promise.resolve(hooks.categorySkillReminder?.event?.(input))
     await Promise.resolve(hooks.interactiveBashSession?.event?.(input))