GitLab Duo CLI takes a task from goal to done

Introduces the `/goal` slash command for GitLab Duo CLI, enabling goal-driven agentic workflows. Details the mechanism of defining a goal and a 'done when' condition, with a separate verifier model iterating until the goal is met or a configured limit is reached. Mentions upcoming Slack agent integration and the underlying capability's extension to various clients. Requires GitLab 19.3+, Duo CLI 9.17.0+, Premium/Ultimate tiers, and Duo Agent Platform enabled.