refactor(config): rename model field to model_name

The configuration field for specifying the model has been renamed from
"model" to "model_name" for better clarity and consistency with the
model_list configuration.

A GetModelName() accessor method has been added to maintain backward
compatibility. Existing configurations using the old "model" field will
continue to work correctly.

This change affects:
- Configuration structure (AgentDefaults struct)
- All references across the codebase
- Documentation in all language variants
- Example configuration files

config/config.example.json

@@ -3,7 +3,7 @@
     "defaults": {
       "workspace": "~/.picoclaw/workspace",
       "restrict_to_workspace": true,
-      "model": "gpt4",
+      "model_name": "gpt4",
       "max_tokens": 8192,
       "temperature": 0.7,
       "max_tool_iterations": 20