Golang Spf13 Viper

This teaches Claude how to use spf13/viper, the Go configuration library that merges flags, env vars, files, and defaults into one key-value API. The skill covers the fixed precedence pipeline (flag beats env beats file beats default), the three-part env setup that everyone gets wrong (SetEnvPrefix plus SetEnvKeyReplacer plus AutomaticEnv), binding cobra flags in init or PersistentPreRunE, and unmarshaling into structs with mapstructure tags. It pushes hard on common bugs: missing ConfigFileNotFoundError handling, skipping the key replacer so nested env vars silently fail, and using the global viper in tests instead of viper.New(). Solid if you're wiring viper into a CLI or daemon and want to avoid the usual pitfalls.