[FEATURE]: Add GitLab Duo Agentic Chat Provider Support

[vglafirov]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

In-line with GitLab's open-core strategy, we want to officially integrate GitLab Duo with OpenCode as a supported provider, operable with both GitLab.com and self-hosted GitLab instances. To streamline our users' journey across tools and enable access to GitLab's AI capabilities including agentic chat with native tool calling, GitLab will be upstreaming the required provider packages to this repository.