Add Audit trail and AI agent inventory dashboard cards

What

Implements two of the AI Governance Dashboard cards with real data, reusing existing GraphQL (no backend change):

• Audit trail — recent agent sessions via duoWorkflowSessionArtifacts (the same field the "Audit events" tab uses), group and project scoped, first: 5. Each row links to the session; "View full audit log" opens the Audit events tab.
• AI agent inventory — configured agents via aiCatalogConfiguredItems (a lean query on the same resolver the agent registry uses), group and project scoped. "View all agents" links to the agents page.

Both cards handle loading, empty, and error states, and pick the group vs project query based on the injected path. A small shared DashboardListCard provides the card chrome.

Scope

• Internal data only. External agents (Cursor, Claude Code, etc. shown in the design) are not supported by the backend yet and are out of scope here (tracked for GAIG Stage 1).
• Placeholders remain for the AI Agents and AI Sessions cards.

Screenshot / Screen Recording

How to verify this locally

1. Set up GitLab Duo locally (SaaS mode + licensed group): see Set up GitLab Duo for local development.
2. Enable the flag: Feature.enable(:ai_governance_dashboard)
3. Visit Settings → GitLab Duo → Governance on the gitlab-duo group and open the Dashboard tab.

References

• Related issue https://gitlab.com/gitlab-org/gitlab/-/work_items/604505
• Planning issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/603776

Stacked on the scaffold MR !243109 (merged); retarget to master once that merges.