Update agent model defaults

[allanisliu-oss]

*** Begin Patch
*** Update File: definitions/explore.agent.yaml
@@
-model: claude-haiku-4.5
+model: gpt-5-mini

*** Update File: definitions/task.agent.yaml
@@
-model: claude-haiku-4.5
+model: gpt-5.4

*** Update File: definitions/code-review.agent.yaml
@@
-model: claude-sonnet-4.5
+model: claude-opus-4.6
*** End Patch