servicepack v1.2.21 — follow slog-configurator to slogging

The logging dependency moved to github.com/psyb0t/slogging/slogconf. Exported
names are unchanged, so nothing about logging behaves differently.

The scaffold's docs move with it -- the README custom-handler example,
cmd/init.go's comment, the dependency list and the .agents/ skill -- since a
generated project copies those.