Clarify env boolean parsing

- Add `boolean: :set` for generated env readers where any non-empty
  value must mean enabled, matching Bash checks and avoiding duplicate
  `_NO_` opt-outs.
- Add `disabled_by:` so generated readers can model one env var
  overriding another while keeping both env vars documented from `ENVS`.
- Use `disabled_by:` for generated enable/disable env pairs and remove
  redundant caller-side checks.
- Inline pass-through helpers that only forwarded to generated
  `EnvConfig` readers.
- Document the generated boolean modes near `FALSY_VALUES` so future env
  vars can choose the correct Ruby and Bash contract.