env/prod: Collect common tempates in module

In order to simplify the process of adding new template render
instructions to all hosts, I've created a list of templates in the
`env/prod` module.  This way, I only have to add templates there, and
all hosts that "inherit" from it will automatically get them.

env/prod/instructions.cue

@@ -0,0 +1,13 @@
+package prod
+
+import (
+	"list"
+
+	"du5t1n.me/cfg/app/ssh"
+	"du5t1n.me/cfg/app/sudo"
+)
+
+templates: list.Concat([
+	ssh.templates,
+	sudo.templates,
+])