Adding Nebul as provider for opencode

[wynandhuizinga]

I would like to add nebul as provider. I'll create a PR for it.

[github-actions]

This is a request to add a new provider, not a concrete catalog data fix (missing/incorrect model metadata). The author also plans to open their own PR, and there’s no verified model/pricing data to apply.

No automated fix was opened — it needs maintainer review.

[wynandhuizinga]

PR: #6961