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/wifi.sh

@@ -1,11 +1,11 @@
 notify_update() {
   omarchy-notification-send -u critical -g  "Update System" "Click to update the system." \
-    --exec "omarchy-launch-floating-terminal-with-presentation omarchy-update"
+    --exec-arg omarchy-launch-floating-terminal-with-presentation --exec-arg omarchy-update
 }
 
 notify_wifi() {
   omarchy-notification-send -u critical -g 󰖩 "Setup Wi-Fi" "Click to configure the wireless network." \
-    --exec "omarchy-shell shell toggle omarchy.network"
+    --exec-arg omarchy-shell --exec-arg shell --exec-arg toggle --exec-arg omarchy.network
 }
 
 announce_network() {