Support spec-level agent and model routing

[udaychandra]

Summary

• add optional spec.yaml agent, model, and reasoning_effort routing
• make CLI and API dispatch prefer spec.agent over project default agent.type
• map Codex model routing to --model and model_reasoning_effort config flags
• reject model/reasoning options for agents that do not support them yet
• document repo/agent/model routing in generated agent context, spec guidance, and README

Tests

• mvn clean verify