feat(ai): add model caching, settings, and commands

- Introduce model caching with persistence to preferences
- Add provider default model and custom settings management
- Implement `/ai switch`, `/ai models`, and improve `/ai start`
- Add model name autocomplete for chat commands
- Update command definitions and help text
- Add unit tests for new functionality

src/config/preferences.h

@@ -366,4 +366,10 @@ char* prefs_ai_get_token(const char* const provider);
 GList* prefs_ai_list_tokens(void);
 void prefs_free_ai_tokens(GList* tokens);
 
+/* AI model cache management */
+gboolean prefs_ai_set_models(const char* const provider, const gchar* const* models, gint count);
+gboolean prefs_ai_remove_models(const char* const provider);
+GList* prefs_ai_get_models(const char* const provider);
+void prefs_free_ai_models(GList* models);
+
 #endif