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/fallback-models.ts

@@ -0,0 +1,5 @@
+import { z } from "zod"
+
+export const FallbackModelsSchema = z.union([z.string(), z.array(z.string())])
+
+export type FallbackModels = z.infer<typeof FallbackModelsSchema>