Chore: Remove duo_panel_history_stack_persistence FF

What does this MR do and why?

This MR removes the duo_panel_history_stack_persistence feature flag and keeps the history-stack persistence behaviour permanently on.

The rollout was paused in July after navigation bugs turned up, restarted on 2026-08-21, and reached true on gprd on 2026-08-27 (https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/work_items/63195), so the cleanup is now unblocked.

References

• Related GitLab Work Item: #601680 (closed)

Screenshots or screen recordings

Screen Recording

How to set up and validate locally

For DAP/General Setup

• General DAP: https://docs.gitlab.com/development/duo_agent_platform/
• Assigning Duo to a group: https://docs.gitlab.com/development/ai_features/#run-gitlabduosetup-script
• Make sure you are in SaaS mode with export GITLAB_SIMULATE_SAAS=1 set in your env.runit file in your local gdk.
• Turn on the following feature flags: duo_workflow, duo_workflow_in_ci, ai_flow_triggers.

Adding Sessions

1. A: If you see no Sessions, you can trigger one that displays a Pipeline/Job by going to an Issue within your Duo enabled project and clicking the Generate MR with Duo button. It should trigger a new session that you can open up.
2. B: You can also run the following query bundle exec rake "gitlab:duo_workflow:populate[50,20,1,<<YOUR_TEST_GROUP_PATH_HERE>>]" to create new agent sessions. Refresh the page. Make sure the group you are testing in has Duo assigned (see DAP setup above).

To Test in new UI Side Panel

• Before pulling this branch, you can test on master with this FF (duo_panel_history_stack_persistence) to test the functionality.
• You may use the following MR for regression testing the navigation.

1. Go to the Side Panel > Sessions > (select any Session)
2. In your left navigation, navigate to a new page (ex: moving from Work Items to Merge Requests)
3. Validate the Sessions navigation still has the back button and remains on the Session detail view

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.

Related to #601680 (closed)