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/oh-my-opencode-config.ts

@@ -14,6 +14,7 @@ import { GitMasterConfigSchema } from "./git-master"
 import { HookNameSchema } from "./hooks"
 import { NotificationConfigSchema } from "./notification"
 import { RalphLoopConfigSchema } from "./ralph-loop"
+import { RuntimeFallbackConfigSchema } from "./runtime-fallback"
 import { SkillsConfigSchema } from "./skills"
 import { SisyphusConfigSchema } from "./sisyphus"
 import { SisyphusAgentConfigSchema } from "./sisyphus-agent"
@@ -52,6 +53,7 @@ export const OhMyOpenCodeConfigSchema = z.object({
   websearch: WebsearchConfigSchema.optional(),
   tmux: TmuxConfigSchema.optional(),
   sisyphus: SisyphusConfigSchema.optional(),
+  runtime_fallback: RuntimeFallbackConfigSchema.optional(),
   /** Migration history to prevent re-applying migrations (e.g., model version upgrades) */
   _migrations: z.array(z.string()).optional(),
 })