Add agent_presence_consolidation feature flag

What does this MR do and why?

Add agent_presence_consolidation feature flag

• Add the agent_presence_consolidation WIP flag, default off, to gate the shared agent presence actor line and session link
• Push the flag to the frontend from the work item, issue, and merge request controllers, scoped to the root namespace

References

• Related to Standardize agent presence and traceability acr... (gitlab-org#22957) • Andrew Fontaine, Emily Bauman+ • 19.6

Screenshots or screen recordings

No UI changes - reasoning in this comment: !255699 (comment 3837778185)

How to set up and validate locally 🤖

1. In the rails console, enable the flag for a top-level group (not a subgroup):

   Feature.enable(:agent_presence_consolidation, Group.find_by_full_path('<top-level-group>'))

2. Visit an issue or work item in a project under that group

3. In the browser console, confirm the flag reached the frontend:

   gon.features.agentPresenceConsolidation; // => true

4. Repeat on a group-level work item and on a merge request

5. Disable the flag and confirm it flips to false:

   Feature.disable(:agent_presence_consolidation, Group.find_by_full_path('<top-level-group>'))

MR acceptance checklist

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