Give the build verb the command's other two shapes, so a changed base or a source that moved can be rebuilt through the console

The verb took only a repository; build --on <module> and build --behind were reachable only through the generic command verb. Each shape is one per call, and a second beside it is refused as passed over.

cmd/mesh-controller/seatverbs.go

@@ -694,8 +694,19 @@ func (a *verbArguments) commandLine() ([]string, error) {
 		}
 		return []string{"module", "issue", str("module"), "--node", str("node")}, nil
 	case "build":
-		if err := need("repository"); err != nil {
-			return nil, err
+		// The command's three shapes (`build --on`, `build --behind`, `build <repository>`), one per
+		// call: each branch reads only its own argument, so another given beside it is refused as
+		// passed over rather than dropped. Asked and not waited for, the same as a single build.
+		if b := str("on"); b != "" {
+			return []string{"build", "--on", b, "--wait", "0"}, nil
+		}
+		if on("behind") {
+			return []string{"build", "--behind", "--wait", "0"}, nil
+		}
+		if a.given["repository"] == "" {
+			// No shape named: the command says its usage, which names all three — the answer the
+			// caller needs, and the same as `rotate` given neither of its shapes.
+			return []string{"build"}, nil
 		}
 		// Not waited for: a tool call cannot hold a connection for the minutes a build takes; the
 		// daemon takes the outcome in when it comes and the id follows the build (issue 176). A