Run notification click actions as argv, not shell strings

The click action of a notification was a free-form shell string run through
`bash -lc`, safe only when every sender shell-quoted every interpolated value
perfectly. One slip is RCE: a hostile yt-dlp video title forged an output
record and injected an mpv option into the click command (mehmetince.net RCE,
partially addressed by #7847).

Add a parameterized transport: omarchy-notification-send gains --exec-arg
(repeatable), encoding a JSON argv into the omarchy-exec-argv hint. The shell
runs it with Quickshell.execDetached(argv) and no shell, so data an attacker
controls is only ever one argument and can never be reparsed as a command. The
shell fails closed on a malformed argv hint.

The legacy free-form --exec string is retained but honored only from Omarchy's
own omarchy-action toasts, and deprecated. Migrate all in-repo callers
(screenshot, screen recording, taildrop receive, migrate-notify, crash-watch,
yt-dlp host) to --exec-arg. Update docs and tests.

bin/omarchy-migrate-notify

@@ -41,7 +41,7 @@ fi
 # The shell keeps the click command with the toast, so this oneshot can hand the
 # invitation over and exit instead of staying activated until it is answered.
 omarchy-notification-send -u critical -g  "Pending Omarchy Migrations" "$message" \
-  --exec "omarchy-launch-floating-terminal-with-presentation omarchy-migrate" && exit 0
+  --exec-arg omarchy-launch-floating-terminal-with-presentation --exec-arg omarchy-migrate && exit 0
 
 # Reached when the notification could not be handed off, so fall back to telling
 # the user in the terminal.