Feat/opencode zen provider

[snail-vs]

Summary

Testing

• [ * ] cargo fmt --all -- --check
• [ * ] cargo clippy --workspace --all-targets --all-features
• [ * ] cargo test --workspace --all-features

Checklist

• [ * ] Updated docs or comments as needed
• [ * ] Added or updated tests where relevant
• [ * ] Verified TUI behavior manually if UI changes
• [ * ] Harvested/co-authored credit uses a GitHub numeric noreply address

[github-actions]

Thanks @snail-vs for taking the time to contribute.

This repository is observing a maintainer-managed PR intake gate in dry-run mode, so this pull request is staying open. This note helps maintainers prepare the allowlist before any enforcement is considered.

Please read CONTRIBUTING.md for the expected contribution shape. A maintainer can grant recurring PR access by commenting /lgtm on a pull request.