Pre-dispatch routing only fills in tasks with no explicit model,
classified by structural signals only (plan > goal > default).
Explicit task model and worker CLI --model win over policy;
policy via CLOUD_ROUTING_POLICY_JSON/FILE + CLOUD_ROUTING_MODE.
Shadow mode records a suggestion without changing dispatch.
Only policy/shadow decisions emit a model_routing event, so the
existing events contract (fallback tests) is unchanged.
Fallback rules, effort/approval/sandbox/quota and provider are
untouched. Tests: test_models_routing.py (20 tests).
