Use duo planner agent for work item summary

What does this MR do and why?

Use duo planner agent for work item summary

Instead of the legacy vue component and using the default Chat agent for summarizing work items, we introduce a new component that can use specific Agents by name. This commit removes the FF to release the feature to all users.

Changelog: added

References

#592935 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable AI features and make sure you have access to both classic and agentic chat
2. Create or navigate to an issue with comments. You will need to have the work_item_planning_view FF set to false
3. Visit the issue page (e.g., http://gdk.test:3000/gitlab-org/gitlab-test/-/issues/1). You know this works when you see "Issues" under the Plan menu and not "Work Item"
4. Verify the "View summary" button appears in the Activity section header with the tanuki-ai icon
5. Click the "View summary" button and verify:
   • The Duo Chat panel opens
   • A new chat session starts
   • The Planner agent is selected
   • The summarization command is queued and executed
6. Repeat steps 3-5 for a work item to verify the component works in both contexts. Turn on the FF work_item_planning_view
7. Verify tracking events are fired when clicking the button (check browser console or monitoring)

MR acceptance checklist

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