Per-Agent Model Override

We should have a way for users to specify different models for each agent. I think the functionality should be to have a default model that all agents use unless a user specifically provides a different one.

Config should likely change to something similar to this:

{
  "models": {
    "default": {  // This 'default' model is required in the config
      "model": "gpt-5.4",
      "api_key": "${API_KEY}",
      "base_url": "${API_BASE_URL:-https://livai-api.llnl.gov/v1}"
    },
    "my_model": {
      "model": "claude-sonnet-3.7",
      "api_key": "${API_KEY}",
      "base_url": "${API_BASE_URL:-https://livai-api.llnl.gov/v1}"
    }
  },
  "agents": [
    {
      "agent_name": "AgentWithDefaultModel",
      "description": "Agent using the default model (no 'model' setting)",
      "domain": "my_domain",
      "mcp_servers": ["some_server"],
      "instructions": "Do something",
    },
    {
      "agent_name": "AgentWithCustomModel",
      "description": "Agent using a custom model (has 'model' setting)",
      "domain": "my_domain",
      "mcp_servers": ["some_server"],
      "instructions": "Do something",
      "model": "my_model"
    }
  ]
}

No files or tests are named. Start by locating the configuration parser and agent/model construction entry points, then trace how the current model is selected. Done means a required default model works for agents without an override and an explicitly named model is used for agents that specify one.