ONBOARDING

Override whether the first-run onboarding flow opens at interactive startup.

Three-state, parsed by classify_env_bool:

• Unset (or an unrecognized token): keep the default first-run behavior, i.e. run onboarding until the completion marker exists.
• Falsy (0, false, no, off, or empty): never open onboarding, even on a fresh install with no completion marker.
• Truthy (1, true, yes, on): force onboarding to open on every interactive startup, ignoring the completion marker.

Read by should_run_onboarding; skipping the flow this way does not write the completion marker, so unsetting the variable restores first-run behavior.