Forward unresolved explicit subagent model selectors into child session startup so modelRoles.task cannot disappear during executor preflight and fall through to an unrelated provider default.\n\nFixes #4421
Kept OpenRouter and Vercel upstream routing suffixes in subagent retry fallback selectors so same-base routed candidates stay distinct.
Resolved retry fallback candidates from the raw selector before model switching so routed fallback models keep their requested upstream route.
Placed subagent-scoped fallback chains ahead of inherited retry chains so overlapping primary selectors prefer the explicit subagent model order.
Expanded the regression to pin fallback chain insertion order when a global chain shares the same primary selector.
Installed subagent-scoped retry fallback chains from ordered task model candidates so provider failures can advance to the next configured worker model.
Updated task result tracking to surface the fallback-applied final model and added focused regression coverage for the executor wiring.
Fixes#2750
Jeff Scott Ward