Update agent todos section in session details

What does this MR do and why?

• Renames the todo_write plan section in the session details from Progress to Agent todos
• Moves the section to the top of the session details panel and removes the border around the step list

References

• Related to Surface todo_write plan state in session detail... (#603503 - closed) • Emily Bauman, Allison Villa • 19.2
• Feedback in this thread: #603503 (comment 3532563970)
• First MR: !241946 (merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Prerequisite: Have GitLab Duo set up in your GDK
   • Have a Duo-enabled group and project in your GDK created by the Duo setup task above
2. Prerequisite: Have GitLab Runner working
3. Prerequisite: Have the duo_session_plan_section feature flag turned on - bundle exec rails runner "Feature.enable(:duo_session_plan_section)"
4. Start a GitLab Duo agent flow session that produces a plan (the agent emits todo_write tool calls).
5. Open the session detail page and confirm the "Plan" section appears in the info sidebar, showing the checklist and the "X of Y completed" summary.
6. Open the same session in the AI side panel and confirm the Plan section also renders there.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.