Add duo_automations feature flag

What does this merge request do and why?

Adds a wip feature flag, duo_automations, for the redesign of the Duo Agent Platform "Triggers" list into "Automations". The flag is off by default, uses the current user as the actor, and only affects the UI. The project Automate controller pushes it to the frontend.

When the flag is on, the Automate page view sets @force_show_panel_header. This keeps the panel header, with its breadcrumbs and the actions area used by PanelActionsPortal, always visible. The frontend page layout guide requires this for pages that put buttons in the panel header. Nothing else changes yet. The next merge requests in the stack use the flag.

References

https://gitlab.com/groups/gitlab-org/-/work_items/23940

Prototype: !259784 (closed)

Stack:

1. !260755 (merged)
2. !260756 (merged)
3. !260757 (merged)

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