Library engines are hermetic by default

A freshly constructed facter.Engine resolves core facts only: it does not read facter.conf, does not scan default custom/external fact directories, does not execute external-fact scripts, and does not touch the persistent cache. All of that is explicit opt-in via functional options (WithConfigFile, WithCustomDirs, WithExternalDirs, WithCache, or WithSystemDefaults for full CLI-equivalent behavior). This deliberately diverges from the CLI, which follows the system automatically — a library that silently executes scripts found in well-known directories on the host is a security footgun, and implicit host config makes embedding consumers’ behavior (and tests) nondeterministic. The input contract is unaffected: custom facts, external facts, and facter.conf all work identically once opted into, and the facter CLI constructs its engine system-following, so CLI behavior is unchanged.

Do not “fix” the library default to match the CLI — the asymmetry is the decision.