Squashed from claude/fervent-booth-fb7b1f. Hardens resolveModelPolicy against prototype pollution and fixes resolveModelForTier to check model_policy before dynamic_routing. 199 tests green.
Squashed from claude/fervent-booth-fb7b1f. Hardens resolveModelPolicy against prototype pollution and fixes resolveModelForTier to check model_policy before dynamic_routing. 199 tests green.