Function resolve_route 

pub fn resolve_route(
    phase: AgentPhase,
    config: &ModelTierConfig,
    requested_tier: Option<ModelTier>,
    budget: ModelBudget,
) -> ModelRoute

Resolve the model route for a phase.

Resolution order:

1. An explicit requested_tier is honored.
2. For Explore, an explorer_model override wins (tier = explorer_tier or Speculator).
3. Otherwise the phase’s default tier applies.