feat(model-registry): add provider field to all entries

Add optional provider field (anthropic, openai, alibaba, github,
openrouter, opencode) as groundwork for F-WE-PDIV provider diversity
enforcement in auto-model selection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

model-registry/gpt-4o-openai.yaml

@@ -1,5 +1,6 @@
 name: gpt-4o-openai
 endpoint: gpt-4o-openai
+provider: openai
 scores:
   complexity: 8
   test_pass_rate: 8