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>>

src/shared/index.ts

@@ -34,6 +34,7 @@ export * from "./system-directive"
 export * from "./agent-tool-restrictions"
 export * from "./model-requirements"
 export * from "./model-resolver"
+export { normalizeFallbackModels } from "./model-resolver"
 export { resolveModelPipeline } from "./model-resolution-pipeline"
 export type {
   ModelResolutionRequest,

src/shared/model-resolver.ts

@@ -7,6 +7,17 @@ export type ModelResolutionInput = {
 	systemDefault?: string
 }
 
+/**
+ * Normalizes fallback_models to an array.
+ * Handles single string or array input, returns undefined for falsy values.
+ */
+export function normalizeFallbackModels(
+	fallbackModels: string | string[] | undefined | null
+): string[] | undefined {
+	if (!fallbackModels) return undefined
+	return Array.isArray(fallbackModels) ? fallbackModels : [fallbackModels]
+}
+
 export type ModelSource =
 	| "override"
 	| "category-default"