feat: add due date to list sorting priority

Sort nodes by: status → priority → due date (soonest first).
Nodes without due dates sort last.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/cmd/output.go

@@ -76,7 +76,21 @@ func PrintNodes(w io.Writer, svc service.NodeService, nodes []*models.Node, json
 		if si != sj {
 			return statusRanks[si] > statusRanks[sj]
 		}
-		return prioRanks[nodes[i].GetProperty("prio")] > prioRanks[nodes[j].GetProperty("prio")]
+		pi, pj := prioRanks[nodes[i].GetProperty("prio")], prioRanks[nodes[j].GetProperty("prio")]
+		if pi != pj {
+			return pi > pj
+		}
+		di, dj := nodes[i].DueDate, nodes[j].DueDate
+		if di == nil && dj == nil {
+			return false
+		}
+		if di == nil {
+			return false
+		}
+		if dj == nil {
+			return true
+		}
+		return di.Before(dj.Time) // soonest due date first
 	})
 
 	for _, n := range nodes {