fix: restore 'opus' as default ID for latest Opus model

Keep backwards compat - users who had 'opus' should get the latest (4.6).
opus-4.5 still available for users who want the specific version.

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src/models.json

@@ -24,7 +24,7 @@
     }
   },
   {
-    "id": "opus-4.6",
+    "id": "opus",
     "handle": "anthropic/claude-opus-4-6",
     "label": "Opus 4.6",
     "description": "Anthropic's best model with adaptive thinking",