fix(plugins): retry claude plugin install|update without -y on an old CLI

An older Claude Code has no -y on `plugin install|update` and exits with
"error: unknown option '-y'", so the dashboard's Update now chain failed at
its plugin_update step (and so did Update all, the plugin buttons and the
superpowers auto-install). Those versions also predate the confirmation
prompt -y answers, so every wrapper now retries the same argv without it
when, and only when, the CLI rejects the flag. /agenthub-update, which the
failure message points users to, falls back the same way.

plugins/agenthub/commands/agenthub-update.md

@@ -17,7 +17,9 @@ if [ -z "$MARKETPLACE" ]; then
   echo "agenthub is not installed as a plugin (checkout install?) - nothing to update this way."
 else
   claude plugin marketplace update "$MARKETPLACE"
-  claude plugin update "agenthub@$MARKETPLACE" -y
+  # An older Claude Code has no `-y` (and no prompt for it to answer).
+  claude plugin update "agenthub@$MARKETPLACE" -y 2>/dev/null ||
+    claude plugin update "agenthub@$MARKETPLACE"
 fi
 ```