Document trigger-based setup for DAP reviewer assignment

Phase B. Written alongside #607677 (closed) and shipped with it — the page is wrong the moment the setting disappears from the UI.

Problem

doc/user/project/merge_requests/reviews/automatic_reviewer_assignment.md documents a Reviewer assignment strategy setting with an Assign reviewers with GitLab Duo Agent Platform radio option. That setting is being removed, and the page has no trigger-based setup instructions.

Scope — specific locations

  • Lines 58-99: remove the Reviewer assignment strategy section, including the history entry at line 62 that names the dap_powered_recommend_reviewers feature flag.
  • Add setup steps for the new path: enable the Recommend Reviewers flow for the group, then create a trigger in Automate > Triggers on Merge request > Marked ready. In the trigger UI this is an action under the consolidated Merge request event (consolidated in 19.2), not a standalone event — match the UI wording.
  • Keep the existing explanation of how candidates are ranked (availability, workload, local time, recent activity, and the All Members rule fallback) — that behaviour does not change.
  • Document the actor/attribution model per #607675 (closed): the assignment and progress notes are authored by the flow's service account; the person who marks the MR ready must have at least the Developer role or the run silently assigns nothing; automation/bot-marked-ready MRs do not trigger the flow.
  • Cross-link the DAP Triggers docs and the composite identity docs.
  • REST API docs (doc/api/projects.md, OpenAPI enums) are handled in #607677 (closed), not here.

Must document the Beta coverage limitation

MergeRequests::ReadyEvent is only published on a draft > ready transition (app/services/merge_requests/update_service.rb). The bespoke path also fired for merge requests opened non-draft, once diffs landed (ee/app/services/ee/merge_requests/reload_merge_head_diff_service.rb plus ReviewerAssignment::PendingInitialAssignment).

So after the migration, a merge request opened directly in a ready state does not get reviewers assigned. The current page states the opposite:

A merge request is created in a ready state.

That line must change for the DAP strategy. The gap closes when the merge_request_created trigger lands (#592452 (closed)), which @phikai scoped to GA.

Acceptance criteria

  • Page describes trigger-based setup, with no reference to the removed setting
  • Beta limitation on ready-on-create merge requests documented explicitly
  • Permission requirement (Developer+ ready-marker) and service-account attribution documented
  • Technical Writing review
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