Run notification click actions as safe argv (backport of #7926)

Backport of the notification click-command hardening (PR #7926, merged to
quattro as 43bfe9b9) onto the v4-0-1 release branch. Click actions are argv
vectors run without a shell, omarchy-notification-send calls the Notify D-Bus
method directly via busctl instead of notify-send, and --exec takes the command
as rest-of-line words. Excludes docs/notifications.md, which does not exist on
v4-0-1.

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 omarchy menu summon setup.default.agent
 fi