Feature flag rollout: duo_agent_sessions_on_board
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## Summary
Rollout issue for the `duo_agent_sessions_on_board` feature flag, introduced in !237105.
This flag gates the session badge on issue board cards that shows Duo Agent session activity.
## What does the flag do?
When enabled, board cards show a compact badge indicating the number of agent sessions attached to a work item. Clicking the badge opens a popover with session details and links to the Duo side panel.
## Rollout plan
1. Enable on staging and verify
2. Enable for internal groups (gitlab-org, gitlab-com)
3. Enable for 50% of groups
4. Enable globally (default on)
5. Remove the feature flag
## Links
- Feature MR: !237105
- Feature issue: #600601
issue