Bring back clipboard watchers that die

Clipboard history is fed by two wl-paste --watch processes started once
when the shell loads. When one dies, nothing notices: copying still
works, the picker still opens, and the old entries are all still there,
so history just silently stops recording until the next shell reload.

Respawn them on exit instead. The one-shot timer both coalesces the case
where both watchers die together and keeps a wl-paste that cannot start
at all from spinning as fast as fork allows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/shell.d/clipboard-test.sh

@@ -174,6 +174,11 @@ assert(
   clipboardQml.includes('command: ["pkill", "-f", "wl-paste .*--watch .*/shell/plugins/clipboard/capture\\\\.sh"]'),
   'clipboard init reaps stale watchers before starting new ones'
 )
+assertEqual(
+  (clipboardQml.match(/onExited: watchRestartTimer\.restart\(\)/g) || []).length,
+  2,
+  'clipboard respawns both watchers when they die'
+)
 JS
 
 TMPDIR=$(mktemp -d)