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/agent-overrides.ts

@@ -1,9 +1,11 @@
 import { z } from "zod"
+import { FallbackModelsSchema } from "./fallback-models"
 import { AgentPermissionSchema } from "./internal/permission"
 
 export const AgentOverrideConfigSchema = z.object({
   /** @deprecated Use `category` instead. Model is inherited from category defaults. */
   model: z.string().optional(),
+  fallback_models: FallbackModelsSchema.optional(),
   variant: z.string().optional(),
   /** Category name to inherit model and other settings from CategoryConfig */
   category: z.string().optional(),