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-fingerprint.hook

@@ -8,5 +8,5 @@ if omarchy-hw-fingerprint && [[ ! -f /etc/pam.d/omarchy-lock-fingerprint ]] &&
   omarchy-done ensure fingerprint-setup-invitation; then
   omarchy-notification-send -u critical -g 󰈷 "Setup Fingerprint Reader" \
     "Enable sudo and unlocking with your fingerprint." \
-    --exec "omarchy-launch-floating-terminal-with-presentation omarchy-setup-security-fingerprint"
+    --exec-arg omarchy-launch-floating-terminal-with-presentation --exec-arg omarchy-setup-security-fingerprint
 fi