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/config/index.ts

@@ -11,6 +11,8 @@ export {
   RalphLoopConfigSchema,
   TmuxConfigSchema,
   TmuxLayoutSchema,
+  RuntimeFallbackConfigSchema,
+  FallbackModelsSchema,
 } from "./schema"
 
 export type {
@@ -29,4 +31,5 @@ export type {
   TmuxLayout,
   SisyphusConfig,
   SisyphusTasksConfig,
+  RuntimeFallbackConfig,
 } from "./schema"