Take only a check build its verdict names, and keep a build another check still names

The artifact store takes writes from anything on its network, so a record
planted under another merge's key could have been registered: the walk now
takes a scratch build only when the build seat's own verdict for one of its
pull requests names its key and digests. Deleting by digest also took builds
another check shared (review of #311; novox/hq ADR 0350).

internal/link/build.go

@@ -101,8 +101,22 @@ type BuildRequest struct {
 }
 
 // ReuseAsk is a walk's leave to take a check's build (novox/hq ADR 0350 §2).
+//
+// **Only a build a verdict names is taken.** Builds are the builds the build seat said, in the outcomes of its checks
+// of the walk's own pull requests, that it made for their merges: each key with its artifacts' digests. The record
+// in the scratch namespace is not trusted alone — the artifact store takes writes from anything on the network, a
+// check runs a pull request's own script on that network, and a record planted under another merge's key would
+// otherwise be registered. The builder takes a scratch build only when a build here names its key and every digest.
 type ReuseAsk struct {
-	Pulls []PullRef `json:"pulls,omitempty"`
+	Pulls  []PullRef      `json:"pulls,omitempty"`
+	Builds []CheckedBuild `json:"builds,omitempty"`
+}
+
+// CheckedBuild is one build a check's verdict names: the pull request it checked, its ask, and the build.
+type CheckedBuild struct {
+	Pull  PullRef    `json:"pull"`
+	Check string     `json:"check"`
+	Built CheckBuilt `json:"built"`
 }
 
 // PullRef names a pull request.