Add todo_write plan to session detail sidebar

What does this MR do and why?

Adds a "Plan" section to the GitLab Duo agent session info sidebar that surfaces the agent's latest todo_write plan via the todo checklist component, so users can see what the agent intends to do and track its progress. The todo list is visible in both the session details in the side panel and the full-page view. Gates this behind a duo_session_plan_section feature flag.

References

• Related to Surface todo_write plan state in session detail... (#603503) • Emily Bauman, Allison Villa • 19.2

Screenshots or screen recordings

Before	After

Recording

- In this recording, the grey border above the plan isn't flush to the left edge of the panel to match the bottom border - this is fixed and reflected in the screenshots no longer applicable

• In the middle I open the Job ID tab by accident - please excuse 😅

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. Enable the FF - 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

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