Machine-specific Fish shell configuration for this Mac (home-manager + Nix). Covers the managed-vs-editable boundary, function authoring with mandatory fish_indent validation and stub testing, macOS Keychain secret loading, PATH layers, and bash-to-fish migration. Use when creating or editing fish functions, aliases, or abbreviations, adding PATH or environment variables, writing or debugging fish scripts, touching config.fish or conf.d files, changing prompt or theme, migrating bash snippets to fish, or when phrases like "alias para la terminal", "funcion de fish", "config.fish", "conf.d", "migrate bash to fish", or "fish_indent" appear.
