Remove --exec entirely; --exec-arg is the only click-command form

A free-form shell-string --exec sitting next to the safe --exec-arg is a
standing invitation for the next caller to interpolate untrusted data and
reintroduce the RCE. Remove it: omarchy-notification-send --exec now errors and
points at --exec-arg, and the shell drops the omarchy-exec string hint and its
bash -lc execution path, leaving only the argv path.

Migrate the remaining string callers (the first-run invitation hooks, wifi and
welcome prompts) to --exec-arg, and update their notification mocks. Trim the
verbose security comments added along the way.

install/user/first-run/setup-agent.hook

@@ -7,5 +7,5 @@ set -e
 if [[ -z $(omarchy-default-agent) ]] && omarchy-done ensure agent-setup-invitation; then
   omarchy-notification-send -u critical -g 󰚩 "Set your default agent" \
     "Let your favorite agent help with Omarchy." \
-    --exec "omarchy menu summon setup.default.agent"
+    --exec-arg omarchy --exec-arg menu --exec-arg summon --exec-arg setup.default.agent
 fi