Name the hold a waiting walk waits on in plans (issue 529)

A walk whose delivery was held for a person said only that it waits for
mesh-delivery's word and that plans go starts it by hand, pointing the
reader at a hand start instead of the person's decision. Ask mesh-delivery
what holds each waiting walk and say the held delivery, its reason and
mesh-delivery.release, or the delivery not at its turn.

cmd/mesh-controller/delivery_conditions.go

@@ -37,6 +37,9 @@ type stalledLine struct {
 	// Number is the pull request's, for a line of a head the forge never announced (novox/hq issue 347).
 	Number int    `json:"number,omitempty"`
 	State  string `json:"state"`
+	// Walk is the walk a line is about: a waiting walk no live delivery holds, or, asked with `walks`, each walk
+	// waiting for the owner's word and what holds it (novox/hq issues 362, 529).
+	Walk string `json:"walk,omitempty"`
 	// Waiting are the merge checks a line of the state `unanswered` waits on, as mesh-delivery says each:
 	// "mesh/merge-gate pending since <UTC time>", "mesh/repo-check never set" (novox/hq issue 438).
 	Waiting []string `json:"waiting,omitempty"`
@@ -125,6 +128,34 @@ func deliveriesStalled(ctx context.Context, conn *nats.Conn, held bool) ([]stall
 	return lines, nil
 }
 
+// walkWaits are, by walk, the lines mesh-delivery says of a walk waiting for its word: what holds it.
+type walkWaits map[string][]stalledLine
+
+// walksWaiting asks the delivery's owner what holds each walk waiting for its word (novox/hq issue 529): its
+// `stalled` asked with `walks`, so a line of `plans` names a held delivery, its reason and the verb that releases
+// it, instead of pointing at a hand start. Nil when no holder is on record or none answers: the line then reads as
+// before. An owner from before this answers its stalled lines, of which only those naming a walk are kept.
+func walksWaiting(ctx context.Context, conn *nats.Conn, held bool) walkWaits {
+	if !held {
+		return nil
+	}
+	raw, err := askDeliveryOwner(ctx, conn, "stalled", map[string]any{"walks": true})
+	if err != nil {
+		return nil
+	}
+	var lines []stalledLine
+	if json.Unmarshal(raw, &lines) != nil {
+		return nil
+	}
+	out := walkWaits{}
+	for _, l := range lines {
+		if l.Walk != "" {
+			out[l.Walk] = append(out[l.Walk], l)
+		}
+	}
+	return out
+}
+
 // stalledObservations are the conditions of what is stalled.
 func stalledObservations(lines []stalledLine) []conditions.Observation {
 	out := make([]conditions.Observation, 0, len(lines))