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/hooks/index.ts

@@ -45,7 +45,7 @@ export { createCompactionTodoPreserverHook } from "./compaction-todo-preserver";
 export { createUnstableAgentBabysitterHook } from "./unstable-agent-babysitter";
 export { createPreemptiveCompactionHook } from "./preemptive-compaction";
 export { createTasksTodowriteDisablerHook } from "./tasks-todowrite-disabler";
+export { createRuntimeFallbackHook, type RuntimeFallbackHook, type RuntimeFallbackOptions } from "./runtime-fallback";
 export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";
 export { createHashlineReadEnhancerHook } from "./hashline-read-enhancer";
 export { createHashlineEditDiffEnhancerHook } from "./hashline-edit-diff-enhancer";
-