Keep a try off the forge and bound how many run (hq issue 686)

Review of #314: a try's start named its commit, which the forge's holder
matches to an open pull request's head and marks pending. Its start now
names no repository or commit. One try per commit and four in all, a try
never ages ahead of deliveries or pull request checks, and its id is
checked before it is a bus subject.

internal/link/queue.go

@@ -427,6 +427,9 @@ func ReadCheckBuilds(ctx context.Context, js *broker.JetStream, seat string, pul
 // safeKey is an id that can be a key, and a subject token, as it is: a build id, never a pattern.
 var safeKey = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
 
+// SafeBuildID says an id can be a subject token as it is: a build's id, never a pattern.
+func SafeBuildID(id string) bool { return safeKey.MatchString(id) }
+
 // Cancelled is what the controller writes for an ask it cancelled.
 type Cancelled struct {
 	At  string `json:"at"`