fix(trail): treat onboarding defaults as working baselines

Onboarding writes the default runner set, then tune tailors in place. The embedded defaults are working minimal prompts — each carries the output contract its adapter expects — so a runner left un-tailored is still functional and committable, not a broken placeholder.

ensureRunnersPresent reports which runners it created; applyTuneWithAgent tracks which were tailored and reports any created-but-untailored ones neutrally ("kept as working defaults … re-run to tailor"), rather than warning against committing them. The --print path likewise notes created defaults are functional as-is. A test asserts every embedded default contains its adapter's output contract, so the defaults stay genuinely working.