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/runtime-fallback.ts

@@ -0,0 +1,11 @@
+import { z } from "zod"
+
+export const RuntimeFallbackConfigSchema = z.object({
+  enabled: z.boolean().default(true),
+  retry_on_errors: z.array(z.number()).default([429, 503, 529]),
+  max_fallback_attempts: z.number().min(1).max(10).default(3),
+  cooldown_seconds: z.number().min(0).default(60),
+  notify_on_fallback: z.boolean().default(true),
+})
+
+export type RuntimeFallbackConfig = z.infer<typeof RuntimeFallbackConfigSchema>