feat(trail): scaffold default runners when tune runs in a repo with none

entire trail tune now doubles as onboarding. In a repo with no .entire/runners/*.json, it offers to create the default set first (interactive confirmation, or --yes for non-interactive/CI runs) and then tailors them as usual — so a single tune --run bootstraps a repo end to end.

The defaults are embedded in the binary (new runnerdefaults package: the 7 canonical runners with correct contract fields + generic templates) rather than generated by the model, so the structural schema (output adapters, result types, runtime/automation) is always valid and only the templates get tailored. Non-interactive runs without --yes error rather than silently scaffolding.

Tests cover the embedded set's validity/completeness and the create/no-op paths of ensureRunnersPresent.