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/schema/hooks.ts

@@ -46,6 +46,7 @@ export const HookNameSchema = z.enum([
   "task-resume-info",
   "stop-continuation-guard",
   "tasks-todowrite-disabler",
+  "runtime-fallback",
   "write-existing-file-guard",
   "anthropic-effort",
   "hashline-read-enhancer",