AI Catalog updates for governance and operations

Introduces event-driven triggers for GitLab Duo Flows, enabling automated execution based on specific GitLab events (merge request conflict, draft to ready, MR approved, work item created). Adds pipeline event filters for specific states (failure, success, cancellation). Implements admin controls to disable custom agents/flows and restrict AI catalog access to group hierarchies. Adds upstream validation for flow configurations before saving. Introduces public beta for AI model allowlisting and default setting for agentic chat. Adds a pattern-based approval tier for local agentic workflow execution.