GitLab Duo Developer enhancements for merge request workflows

GitLab Duo Developer now supports multiple trigger methods: assign it to an issue, select Generate MR, or @mention it in any issue or MR discussion thread to turn feedback, To-do items, and design questions into code changes, follow-up MRs, or research summaries.

With AGENTS.md and agent-config.yml configured, GitLab Duo Developer runs your tests and checks before committing. After a top-level group or instance administrator enables the Developer Flow, GitLab automatically adds mention and assign triggers to eligible projects.