refactor(api)!: remove Delete method from public API

Environment variables are unaffected by Delete, making it misleading —
callers would expect a key to be fully gone but env vars would still
resolve. Sub is the better abstraction for scoped config.

default.go

@@ -88,10 +88,6 @@ func SetString(key string, value string) {
 	defaultConfigManager.SetString(key, value)
 }
 
-func Delete(key string) {
-	defaultConfigManager.Delete(key)
-}
-
 func Sub(key string) *ConfigManager {
 	return defaultConfigManager.Sub(key)
 }