Add stackctl for running stacks by hand on a host

One shared script installed into /srv/stacks on every host, with per-host
groups in server/<host>/stack-groups. Tested in check.sh. Also two new
common mistakes in the adding-a-stack skill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/edge/stack-groups

@@ -0,0 +1,18 @@
+# Groups for stackctl on edge, installed next to it in /srv/stacks. One group
+# per line:
+#
+#   name: stack stack ...
+#
+# A name on several lines collects all of them. A stack may be in any number of
+# groups, or none; "all" always means every stack with a compose file. Each host
+# has its own file, because each host has its own stacks.
+#
+# Two names are reserved:
+#   order:   started first, in this order; everything else follows
+#            alphabetically, and down runs the whole list in reverse.
+#   confirm: down and restart ask before touching these, unless given -y.
+
+order: reverse-proxy
+
+proxy: reverse-proxy
+monitoring: metrics