Rewrite moduledoc, README and Schema guide examples so each shows the
enclosing modules, aliases, config and calling code. The guide is now a
continuous runnable walkthrough with real results; README and guide
examples were executed in a scratch project to verify them.

Fix Config.Helper.get_env/2 crashing for unset non-string types with no
default (returns nil); document that validations without a default make
a variable required, and that delegates must compile first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>