Try a pushed branch on the build seat before its pull request (hq issue 686)

Store tests skip without a store and no agent may start one, so each failure
cost a 15-30 minute review round. The try verb asks the build seat for the
repository's merge-check.sh on the ref merged into the trunk, as a check in
the build queue, and reads its outcome from the build's log: no status on
the forge, nothing registered, nothing built for a merge.

cmd/mesh-controller/main.go

@@ -106,6 +106,8 @@ func run() error {
 		return killCommand(ctx, args[1:])
 	case "forget-checks":
 		return forgetChecksCommand(ctx, args[1:])
+	case "try":
+		return tryCommand(ctx, args[1:])
 	case "pause", "resume":
 		return pauseCommand(ctx, args[0], args[1:])
 	case "collection":
@@ -381,6 +383,13 @@ func (b builds) Built(ctx context.Context, result link.BuildResult) error {
 	// **A merge check builds nothing** (novox/hq to-be 45 §9): its verdict is said, and nothing of it is
 	// recorded or registered.
 	if result.Check != nil || result.Checked != nil {
+		if isTry(result) {
+			// A branch tried before its pull request (novox/hq issue 686): its asker reads the outcome from the
+			// build's log. Never said as `checked`, so the forge's holder sets no status.
+			fmt.Printf("%s: a try of %s at %.8s, read with `try --id`: said to nobody, nothing taken in\n", result.ID,
+				result.Repository, result.Ref)
+			return nil
+		}
 		checked(ctx, result)
 		return nil
 	}