Let a drain lapse, bound the gate's wait on the controller's clock, keep a busy wait out of apply time

Review of #283: a drain whose replacement was withdrawn held the agent for
ever with its health green; the gate trusted the engine's until; a machine
of the rest waiting at the bound read as silent and its apply time held the
wait; an unknown mesh-builder word exited 0; plan said nothing of a busy an
older engine is not sent; and a node with a report and no send broke the
reading of every last report (the store test's NULL).

cmd/mesh-builder/main.go

@@ -92,8 +92,12 @@ func run() error {
 		case "build":
 			return buildOnce(context.Background(), os.Args[2:])
 		case "ready":
-			// The container's health check (novox/hq issue 544): exits non-zero until the agent takes work.
-			return ready()
+			// The container's health check (novox/hq issue 544): exits non-zero until the agent takes work, and
+			// once it has been drained longer than its node-engine would wait to replace it (ADR 0335).
+			if err := ready(); err != nil {
+				return err
+			}
+			return drainedTooLong(time.Now())
 		case "busy":
 			// The container's busy command (novox/hq ADR 0335): drains the agent, and exits non-zero once
 			// nothing is in hand.
@@ -103,9 +107,14 @@ func run() error {
 			}
 			fmt.Println(said)
 			return nil
-		default:
+		case "help", "-h", "--help":
 			fmt.Print(usage)
 			return nil
+		default:
+			// **An ask this build does not know fails** (novox/hq ADR 0335): an image older than `busy`, asked
+			// it, must read as idle — exiting 0 would hold its container for the node-engine's whole bound.
+			fmt.Fprint(os.Stderr, usage)
+			return fmt.Errorf("%q is not a command this mesh-builder knows", os.Args[1])
 		}
 	}