Allow users to customize the location of their private overlay directory
via `set -U __fish_user_dots_path /your/path`. Falls back to the previous
hardcoded default of `$XDG_CONFIG_HOME/.user-dots/fish` when unset.
Simplify the sourcing model: config.fish now sources only local.fish,
which is responsible for sourcing its own secrets.fish companion. The
redundant direct sourcing of secrets.fish from config.fish is removed.
Update docs/fish-config.md (Sections 1, 7, 10) and README.md to
document the new variable, the custom-path override pattern, and the
simplified sourcing model.
rootiest
