Better constants

[agentcooper]

Improve the code generator, so that well-known constants from schema.json become available.

For example, see SessionConfigOptionCategory:

    "SessionConfigOptionCategory": {
      "anyOf": [
        {
          "const": "mode",
          "description": "Session mode selector.",
          "type": "string"
        },
        {
          "const": "model",
          "description": "Model selector.",
          "type": "string"
        },
        {
          "const": "thought_level",
          "description": "Thought/reasoning level selector.",
          "type": "string"
        },
        {
          "description": "Unknown / uncategorized selector.",
          "title": "other",
          "type": "string"
        }
      ],
      "description": "..."
    },

Generated as:

type SessionConfigOptionCategory string

const (
	SessionConfigOptionCategoryMode         SessionConfigOptionCategory = "mode"
	SessionConfigOptionCategoryModel        SessionConfigOptionCategory = "model"
	SessionConfigOptionCategoryThoughtLevel SessionConfigOptionCategory = "thought_level"
)