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/install-voxtype.hook

@@ -5,5 +5,5 @@ set -e
 if omarchy-done ensure voxtype-install-invitation; then
   omarchy-notification-send -u critical -g  "Install Dictation with Voxtype" \
     "Click to install voice dictation for Omarchy." \
-    --exec "omarchy-launch-floating-terminal-with-presentation omarchy-voxtype-install"
+    --exec-arg omarchy-launch-floating-terminal-with-presentation --exec-arg omarchy-voxtype-install
 fi

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

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

install/user/first-run/welcome.sh

@@ -2,4 +2,4 @@
 # elides past three lines.
 omarchy-notification-send -u critical -g  "Learn Keybindings" \
   $'Super + K for cheatsheet.\nSuper + Space for Omarchy Menu.' \
-  --exec omarchy-menu-keybindings
+  --exec-arg omarchy-menu-keybindings

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() {