Pipeline monitor

M19 — Reshape the monitor from a per-task log viewer into an issue-centric pipeline view modelled on GitLab CI + Airflow. /issues/pipeline endpoint derives stage state from existing task_history + PR/CI signals. UI: pipeline list (main), expanded per-issue graph, grid view, Gantt timeline, stall detection + round counter + force-merge badges. M18-3 parity view is preserved as the task-event drill-down at /app/monitor/task/:id.

No due date
100% Completed