Announce the build agent's verbs, let a waiting build hear its cancel, and retry a stopped rollout (hq ADR 0219)

The holder's verbs were served and found by nothing; it now answers discovery
with its machine's four, as a runtime announces a seat's verb, so the console
finds <node>/node-build-agent.kill. A cancel publishes no outcome, so a build
waited for also looks at the cancelled set. A plan that stopped at its first
machine is retried by sending that machine the module again, unless a newer
plan holds it.

cmd/mesh-builder/main.go

@@ -141,6 +141,13 @@ func run() error {
 			return err
 		}
 		defer stopServing()
+		// And says so, for the console to find (novox/hq ADR 0197).
+		stopAnnouncing, err := link.OverNATS{Conn: js.Conn()}.Announce(
+			announcement(seat, moduleOf(credential.User), on), log.New(os.Stderr, "", 0))
+		if err != nil {
+			return err
+		}
+		defer stopAnnouncing()
 		go h.stateWhilePaused(ctx, time.Hour)
 	}