Keep a first merge whose manifest word the running build-agent does not read, and name it (issue 1173)

ADR 0482 section 5: a merged manifest the running reader refuses waits for the newer reader, never failed.

internal/inventory/batches.go

@@ -114,6 +114,16 @@ func (i *Inventory) AnswerMerge(ctx context.Context, repository, commit, plan st
 	return i.chainSavePlans(ctx, c, nil, []MergeMove{{Repository: repository, Commit: commit, Plan: plan, Alone: alone}})
 }
 
+// HoldMerge puts a merge back to waiting, no plan answering it, with the event it is kept under replaced (novox/hq issue 1173:
+// a first merge held for a build-agent that reads its manifest, the hold said in its event).
+func (i *Inventory) HoldMerge(ctx context.Context, repository, commit string, event json.RawMessage) error {
+	c, err := i.theChain(ctx)
+	if err != nil {
+		return err
+	}
+	return i.chainSavePlans(ctx, c, nil, []MergeMove{{Repository: repository, Commit: commit, Event: event}})
+}
+
 // Batches is every batch not yet cut: assembling or queued, oldest first. One at a time is the rule; more
 // are read so a fault is seen.
 func (i *Inventory) Batches(ctx context.Context) ([]Plan, error) {

internal/inventory/chain_rollouts.go

@@ -758,6 +758,9 @@ func (i *Inventory) chainSavePlans(ctx context.Context, c *chain.Chain, plans []
 		if m.Plan == "" {
 			f := w.m
 			f.Waiting, f.Alone = true, m.Alone
+			if m.Event != nil {
+				f.Event = m.Event
+			}
 			editOf(w.plan).flag[key] = f
 			continue
 		}

internal/inventory/plans.go

@@ -2,6 +2,7 @@ package inventory
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"slices"
@@ -677,6 +678,8 @@ func (i *Inventory) SavePlan(ctx context.Context, p *Plan) error {
 type MergeMove struct {
 	Repository, Commit, Plan string
 	Alone                    bool
+	// Event replaces the merge's kept announcement when it is put back to waiting (Plan empty); nil leaves it.
+	Event json.RawMessage
 }
 
 // SavePlansMoving writes plans, as SavePlan does each, and gives merges to the plans that answer them, all in one