Update AI reorg metadata (batch 3): 3 remaining files

What does this MR do?

Updates the stage and group frontmatter metadata in 3 documentation files to reflect the AI reorganization:

File	stage change	group change
doc/development/ai_features/availability.md	AI-powered → Security Governance	AI Framework → AI Control Plane
doc/development/agent_principles/distillation_flow.md	AI-powered → Agent Foundations	AI Framework → unassigned
doc/user/duo_agent_platform/flows/claude-edit-v1-flow-registry.md	AI-powered features → Agent Foundations	Workflow Catalog → AI Catalog

This is batch 3 of the AI reorg metadata updates, implementing the mapping suggested in gitlab-org/technical-writing/team-tasks#1685 (comment 3876522924).

References

• Tracking issue: gitlab-org/technical-writing/team-tasks#1685 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.