Make --exec take the command as rest-of-line words

Replace --exec-arg with an ergonomic --exec that consumes the rest of the line
as the click command. The caller's shell tokenizes the words into discrete
arguments before the tool sees them, and the shell runs them as positional
parameters (never a re-parsed string), so safety is identical to the argv form
while the call sites read naturally: `--exec omarchy toggle something`.

Crucially the tool never splits a string itself — a single quoted whole-command
argument is rejected and points at the unquoted form, because whitespace-
splitting a string hands argument boundaries to whoever controls its content
(the injection we are avoiding). --exec must come last; migrate every caller.

test/shell.d/migrate-notify-test.sh

@@ -158,7 +158,7 @@ exec {foreign_lock_fd}>&-
 rm -f "$test_tmp/notify-args"
 run_notify 1 >/dev/null 2>&1
 notify_args_written || fail "migration notifier sends the notification before exiting"
-grep -Fx -- '--exec-arg' "$test_tmp/notify-args" >/dev/null ||
+grep -Fx -- '--exec' "$test_tmp/notify-args" >/dev/null ||
   fail "migration notifier attaches the click command to the toast"
 grep -Fx 'omarchy-launch-floating-terminal-with-presentation' "$test_tmp/notify-args" >/dev/null &&
   grep -Fx 'omarchy-migrate' "$test_tmp/notify-args" >/dev/null ||