fix: inherit LLM provider from global config in setup wizard

[tomasmach]

Summary

• Fix setup wizard creating agents without the correct LLM provider, causing them to use the wrong API endpoint
• Inherit provider field from global config when GLM key and model are configured
• Simplify wizard final step response handling (direct UPDATE_MESSAGE with follow-up fallback)
• Remove debug logging that was added during investigation

Test plan

• Run /init wizard on a server with GLM configured globally — verify the created agent has provider = "glm"
• Run /init wizard with OpenRouter config — verify no provider is set (default behavior)
• Verify wizard completion message displays correctly