sn-update

Refresh installed sn-* skills from upstream SenseNova-Skills.

Decide the scope

• No list given → every sn-* skill upstream.
• Specific skills named → only those. Don't expand.
• A named skill missing upstream → surface as error and stop.
• User said "force / 强制" → re-install even when up-to-date.

Decide the target agent

Check which directories exist: