feat: add foundational analytics agent

What does this merge request do and why?

Add foundational analytics agent flow config.

I moved the prompt from the staging agent in the ai catalog: https://gitlab.com/explore/ai-catalog/agents/536/

Companion Rails MR: gitlab-org/gitlab!210291

Related to Move Duo Analytics Agent from AI Catalog to Fou... (gitlab-org/gitlab#578342)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Checkout changes from !3776 and gitlab-org/gitlab!210291
• Enable FF: foundational_analytics_agent
• Start a new agentic chat with Analytics Agent
• Try a query e.g. 'how many mrs were merged last week'

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.