chore: add AGPL3-or-later license

This CL adds the AGPL3-or-later license for all public code.

It also adds some configuration so we can use headroom to automatically
generate the license headers.

GitOrigin-RevId: 634827ab248c207e6841dbad29f59aeadcbef541
diff --git a/.headroom-templates/html.mustache b/.headroom-templates/html.mustache
new file mode 100644
index 0000000..3d149c8
--- /dev/null
+++ b/.headroom-templates/html.mustache
@@ -0,0 +1,18 @@
+<!--
+  {{ project }}
+  Copyright (c) {{ year }} {{ author }}
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU Affero General Public License as
+  published by the Free Software Foundation, either version 3 of the
+  License, or (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU Affero General Public License for more details.
+
+  You should have received a copy of the GNU Affero General Public
+  License along with this program.
+  If not, see http://www.gnu.org/licenses/.
+-->