fix(runtime-fallback): address cubic AI review issues

- Add normalizeFallbackModels helper to centralize string/array normalization (P3)
- Export RuntimeFallbackConfig and FallbackModels types from config/index.ts
- Fix agent detection regex to use word boundaries for sessionID matching
- Improve tests to verify actual fallback switching logic (not just log paths)
- Add SessionCategoryRegistry cleanup in executeSyncTask on completion/error (P2)
- All 24 runtime-fallback tests pass, 115 delegate-task tests pass

src/shared/model-resolver.ts

@@ -73,3 +73,13 @@ export function resolveModelWithFallback(
 		variant: resolved.variant,
 	}
 }
+
+/**
+ * Normalizes fallback_models config (which can be string or string[]) to string[]
+ * Centralized helper to avoid duplicated normalization logic
+ */
+export function normalizeFallbackModels(models: string | string[] | undefined): string[] | undefined {
+	if (!models) return undefined
+	if (typeof models === "string") return [models]
+	return models
+}