swarm: inherit coordinator model by default; treat swarm_model=inherit/coordinator/empty as inherit

Spawned swarm agents were being pinned to agents.swarm_model when set,
which surprised users whose config still had a stale concrete model
(e.g. openai/gpt-5.4@OpenAI), so workers ran a different model than the
coordinator. resolve_swarm_spawn_model_and_provider now treats empty
strings and the 'inherit'/'coordinator' sentinels as 'no override',
falling back to the coordinator's model+provider. Adds spawn-time logging
of the resolved model and tests covering the sentinels.