feat(review): typed fresh-context review gate (#1623) (#1637)

Adds ReviewGate stage that hard-asserts reviewer runs in a new session
with no implementer transcript priming and returns a three-valued
verdict (pass/fail/questions). Only pass permits auto-merge; fail and
questions both block. ModelSelection enum makes the distinct-reviewer
rule a property of configuration; DifferentModelRequired raises
EvalGateConfigError rather than silently falling back to the same model.