treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/cloud/shepherd/manager/test_agent/main.go b/cloud/shepherd/manager/test_agent/main.go
index 8f29c30..98daf1b 100644
--- a/cloud/shepherd/manager/test_agent/main.go
+++ b/cloud/shepherd/manager/test_agent/main.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // test_agent is used by the Equinix Metal Manager test code. Its only role
 // is to ensure successful delivery of the BMaaS agent executable to the test
 // hosts, together with its subsequent execution.