refactor(shared): add normalizeFallbackModels utility function

Add shared utility to normalize fallback_models config values.

Handles both single string and array inputs consistently.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/types.ts

@@ -100,6 +100,7 @@ export type AgentName = BuiltinAgentName
 export type AgentOverrideConfig = Partial<AgentConfig> & {
   prompt_append?: string
   variant?: string
+  fallback_models?: string | string[]
 }
 
 export type AgentOverrides = Partial<Record<OverridableAgentName, AgentOverrideConfig>>