Default to gpt-image-2 for OpenAI image generation

Make gpt-image-2 the default image-generation model and add it to the
recognized model-id mapping, updating the sample provider configs to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

etc/app/config.yml.dist

@@ -111,7 +111,7 @@ agents:
     #       speed: 1.0
     #       response_format: opus
     #     image_generation:
-    #       model_id: gpt-image-1.5
+    #       model_id: gpt-image-2
     #       style: null
     #       size: null
     #       quality: null